
dass_ 

Book -z 



n 



7776 



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iv.iE'wiiiEsnsiisfiiiss'So 



The following Works, by the author of this volume, 
are for sale at Marsh, Capen & Lyon's Bookstore, No. 
133 Washington Street. 

^voQVtuuibz IBvtvtiwn in SEitfiltsi) 
Qtomponition, 

This popular work was first published about four 
years ago, during which time TWELVE large edi- 
tions of it have been printed in this city, and SIX 
editions in London. It was introduced into the public 
Schools of Boston, soon after its publication ; and it is 
now the only work on composition authorized to be 
used in them. 

^voQttHUibz lEvtvtiwn in 22nj&ItsJ) 
(Grammar* 

PART I. containing the Analysis, and PART II. 
the Synthesis of the English Language. 

This work is also used in the public Schools of this 
city. It has passed through four editions in this coun- 
try, and two in England. It is also very extensively 
used in many public and private Seminaries. 

^voQvtnnibz IBvwtintu in JXf\ttovital 
l&eatrtug, 

Particularly designed to familiarize the younger class- 
es of readers with the pauses, and other marks in 
general use ; and to introduce them to the practice of 
modulation and inflection of the voice. 

This work has been adopted in many respectable 
Seminaries; and has also been Republished in Eng- 
land. 



© ' = © 



*?*e tr*S- THE • £~ • y>**^±^^ 

BOSTON SCHOOL COMPENDJUM 




NATURAL AND EXPERIMENTAL 



PHILOSOPHY, • 



*y 



EMBRACING THE ELEMENTARY PRINCIPLES OF 



MECHANICS, 




OPTICS, 


PNEUMATICS, 




ASTRONOMY, 


HYDRAULICS, 




ELECTRICITY, 


HYDROSTATICS, 




GALVANISM, 


ACOUSTICS, 




magnetism, and 


PYRONOMICS, 




ELECTRO-MAGNETISM \ 


WITH A 


DESCE 


IPTION OP THE 



STEAM AND LOCOMOTIVE ENGINES. * . 

&*? a4~ 7Cr$*jL. fo£- P 'til 



BY RICHARD GREEN PARKIER, A. M. 

Principal of the Johnson Grammar School, Boston, (late Principal of the 
Franklin School,) author of" Progressive Exercises in English Com- 
position," "Progressive Exercises in English Grammar," 
" Progressive Exercises in Rhetorical Reading," &c. 



" Delectando pariterque monendo, 
"Prodesse quam conspici." 



BOSTON: 

MARSH, GAPEN & LYON 

1837. 









*S 






•». 



. i 



><-•-. % 



y<] 



\s 



*1 



Entered, according to Act of Congress, in the year 1837, by 

aUtcfjartt d&xzm $arfter, 
in the Clerk's Office of the District Court of Massachusetts. 



J/f? 



> i* ! 



Printed bt William A. Hall &. Co. 



To the Honorable 

Samuel <Mtkinn SHltot, 

Mayor of the City of Boston, 
and 
Chairman of the School Committee. 
SIR, 

The public Schools of this City are under many obligations 
to you, for the interest you have taken in them, and for your 
disinterested exertions for their improvement. This volume, 
designed to supply a want which they have long felt, affords 
an opportunity of acknowledging the obligation, which I 
gladly embrace. The gratification which I feel in seeing you 
at the head of our municipal institutions, I beg leave to ex- 
press in borrowed language: — 

Tibi ut gratuler non est in animo; sed contra, hanc occa- 
sionem, mihi sic oblatam, nostram civitatem gratulandi, re- 
niti non possum. GLuse omnia solita tua benevolentia ut accip- 
ias quaeso. 

I am, Sir, 

Very respectfully, 

Your obedient Servant, 



PREFACE 



The School Committee of the City of Boston having recently fur- 
nished the Grammar Schools with apparatus for exemplifying the 
principles of Natural Philosophy, the author of this work, who, for ten 
years, has been at the head of one of these large establishments, and 
has felt the want of an elementary treatise unencumbered with extra- 
neous matter ', has been induced to attempt to supply the deficiency. 
If he is not deceived in the result of his labors, the work will com- 
mend itself to notice, by the following features : 

1. It is adapted to the present state of natural science ; embraces 
a wider field, and contains a greater amount of information on the 
respective subjects of which it treats, than any other elementary 
treatise of its size. 

2. It contains an engraving of every article in the Boston School 
set of philosophical apparatus ; a description of each instrument, 
and an account of the experiments which can be performed by 
means of the apparatus. 

3. It is enriched by a representation and a description of the Lo- 
comotive, as well as the common Steam Engine. 

4. Besides embracing a copious account of the principles of Elec- 
tricity and Magnetism, its value is enhanced by the introduction of 
the science of Pyronomics, together with the new science of Elec- 
tro-Magnetism. 

5. It is peculiarly adapted to the convenience of study and of 
recitation, by the figures and diagrams being first placed, side by 
side with the illustrations, and then repeated on separate leaves at 
the end of the volume. The page and the number are also given, 
where each principle may be found, to which allusion is made, 
throughout the volume. 

6. It presents the most important principles of science in a larger 
type ; while the illustrations, and the deductions from these princi- 
ples are contained in a smaller letter. Much useful and interesting 
matter is also crowded into notes at the bottom of the page. By this 



6 PREFACE. 

arrangement, the pupil can never be at a loss to distinguish the parts 
of a lesson which are of primary importance ; nor will he be in 
danger of mistaking theory and conjecture for fact. 

7. It contains a number of original illustrations, which the author 
has found more intelligible to children, than those with which he 
has met elsewhere. 

8. Nothing has been omitted, which is usually contained in an el- 
ementary treatise. 

A work of this kind, from its very nature, admits but little origi- 
nality. The whole circle of the sciences consists of principles de- 
duced from the discoveries of different individuals, in different 
ages, thrown into common stock. The whole, then, is common 
property, and belongs exclusively to no one. The merit, therefore, 
of an elementary treatise on natural science must rest solely on the 
judiciousness of its selections. In many of the works from which 
extracts have been taken for this volume, the author has found the 
same language and expressions without the usual marks of quotation. 
Being at a loss, therefore, whom to credit for some of the expres- 
sions which he has borrowed, he subjoins a list of the works to 
which he is indebted, with this general acknowledgment; in the 
hope that it may be said of him as it was once said of the Mantuan 
Bard, that " he has adorned his thefts, and polished the diamonds 
which he has stolen." 

The thanks of the author are due to Dr. J. W. "Webster, Pro- 
fessor of Chemistry, in Harvard University, for the exhibition and 
explanation of a new and highly interesting apparatus in the de- 
partment of Electro-Magnetism, to which allusion is made in the 
body of this work. 

It remains to be stated, that the Questions, at the bottom of the 
page, throughout the volume, were not written by the author, but 
were prepared by one of the teachers of the school with which he 
is connected. 

12 Orange Street, April, 1837. R. G. P. 



LIST OF WORKS 

Which, have been consulted, or from which extracts have been taken, 
in the preparation of this volume. 

Annals of Philosophy ; Arnott's Elements of Physic ; Bigelow's 
Technology; Cambridge Physics; Chambers' Dictionary; En- 
field's, Olmsted's, Blair's, Bakewell's, Draper's, Grand's, Jones', 
Comstock's, and Conversations on Natural Philosophy ; Franklin's 
Philosophical Papers ; Henry's Chemistry ; King's Manual of Elec- 
tricity ; Lardner on the Steam Engine ; Library of Useful Knowl- 
edge ; Paxton's Introduction to the Study of Anatomy ; Pambour 
on Locomotive Engines on Railways ; Phillips' Astronomy; Silli- 
man's Journal of Science; Singer's Electricity; Scientific Class 
Book ; Scientific Dialogues ; Smith's Explanatory Key ; The Year 
Book; Turner's Chemistry ; Wilkins' Astronomy; Worcester's 
and the American School Geography. 



ERRATA. 

Page 35, No. 123, for " to which a revolving body is confined." read " around 
which alltheparts of a body move." Page 36, No.133, for "round " read " around." 
Page 33, No. 199, in the illustration of fig. 13, it should be slated that '■ the con- 
stantly increasing force nf gravitation, not the resistance of the air, brings the ball 
to E." Page 41, No. 146, after the words " in the same point," insert " with the 
centre of magnitude." Page 46, dele the first note. Page 57, No. 184, for "male 
and female" read " convex a.w\concave ;" make, also, the same correction in the 
illustration. Page 71, No. 217, for li ascertainad " read "ascertained." Page 
76,No. 233,niiistration— for "fig 58,"' read " fig.59," and in the third note, on the 
same pase, transpose the words " longer nnd shorter." Page 78, No. 240, for "uni- 
form" read " aeriform." Page 133, No. 352, insert the following: " That part 
of the Science of Optics which treats of refracted light, is called Dioptrics. Page 
166, 38th line, for "fluids" read " fluid." Page 194, in figure 194, for " Sagita- 
rius " read - l Sagittarius." Page 193, No. 455, dele the words " orbit of the earth 
is called the ecliptic. In other words, the ; " and in fig. 135, page 195, for " Mur- 
cury " read " Mercury." 



NATURAL PHILOSOPHY. 

•SECTION I. 

Divisions of the Subject. 

1. Natural Philosophy is the science which treats 
of the powers and properties of natural bodies, their 
mutual action on one another, and the laws and opera- 
tions of the material world. 

The principal branches of Natural Philosophy, are 
Mechanics, Pneumatics, Hydraulics, Hydrostatics, 
Acoustics, Pyronomics, Optics, Astronomy, Electricity, 
Galvanism, Magnetism, and Electro-Magnetism. 

2. Mechanics is that branch of Natural Philosophy 
which relates to motion and the moving powers, their 
nature and laws, with their effects in machines, &c. 

3. Pneumatics treats of the nature, properties, and 
effects of air. 

4. Hydraulics treats of the motion of fluids, partic- 
ularly of water ; and the construction of all kinds of 
instruments and machines for moving them. 

5. Hydrostatics treats of the nature, gravity, and 
pressure of fluids. 

6. Acoustics treats of the nature and laws of sound. 

1. What is Natural Philosophy? What are the principal branches of Nat- 
ural Philosophy ! 2. What is Mechanics ! 3. Of what does Pneumatics treat J 
4. Hydraulics? 5. Hydrostatics? 6. Acoustics? 

2 



6 NATURAL PHILOSOPHY. 

7. Pyronomics treats of heat, the laws by which it 
is governed, and the effect which it produces. 

8. Optics treats of light, c,olors, and vision, or sight. 

9. Astronomy treats of the heavenly bodies, such as 
the sun, moon, stars, comets, planets, &c. 

10. Electricity treats of thunder and lightning, and 
the causes by which they are produced, both naturally 
and artificially. 

11. Galvanism is a branch of Electricity. 

12. Magnetism treats of the properties and effects 
of the magnet, or loadstone. 

13. Electro-Magnetism treats of the combined pow- 
ers of Electricity and Magnetism. 



SECTION II. 

Of Matter and its Properties. 

14. Matter is the general name of every thing that 
occupies space, or has figure, form or color. 

The words substance, body, or bodies, are but different names for 
the same thing, and they are all comprehended under the general 
name of matter. 

15. All matter is composed of very minute particles, 
which are connected together in different bodies, by 
different, degrees of cohesion. 

16. Those bodies in which these particles strongly 
adhere are hard bodies; and those in which the cohesion 
is not strong are soft. It is therefore owing to the dif- 
ferent degrees of cohesion, that some bodies are hard 
and others soft. 

17. Matter exists in two forms, namely, a solid and 
a fluid form. 

7. Of what Pyronomics ? 8. Optics .' 9. Astronomy: 10. Electricity.' 11. Of* 
what is Galvanism a branch > 12. Of what does Magnetism treat .' 13. Electro- 
Magnetism.' 14. What is Matter.' What is meant by the words substance, 
body, or bodies .' 15. Of what is all matter composed ' How are these particles 
connected together .' 16. What bodies are bard.' What soft .' To what is it 
owin« tbat some bodies are hard and others soft ' 17. In how many forms does 
matter exist .' What are they .' 



MATTER AND ITS PROPERTIES. 7 

18. Matter exists in a solid form when the particles 
ofwhichitis composed adhere together, so that one 
particle cannot be moved without moving the whole. 

19. Matter exists in a fluid form when the particles, 
having bat a slight degree of cohesion move easily 
among themselves. 

20. There are seven essential properties belonging 
to all matter, namely: 1. Impenetrability, 2. Ex- 
tension, 3. Figure, 4. Divisibility, 5. Indestructi- 
bility, 6. Inertia, and 7. Attraction. 

These are called essential properties, because no particle of mat- 
ter can be deprived of them, or exist without them. 

21. There are certain other properties existing in 
different bodies, called accidental properties, because 
they do not necessarily exist in the bodies themselves, 
but depend upon their connexion with other bodies. 
Thus, color and weight are accidental properties, be- 
cause they do not necessarily exist in the bodies that 
possess them, but depend upon their connexion with 
other things. [See Gravity and Optics.] 

22. There are also certain terms used in Natural 
Philosophy, to express the state in which matters ex- 
ists, such asPorosity, Density, Rarity, Compressibility, 
Expansibility, Mobility, Elasticity, Brittleness, Mallea- 
bility, Ductility and Tenacity. 

23. By Impenetrability is meant the power of occu- 
pying a certain space, so that where one body is, anoth- 
er cannot be, without displacing it ; because two bod- 
ies, or two portions of matter cannot occupy the same 
space at the same time. 

24. Impenetrability belongs to fluids as well as solid 
bodies ; and it is as impossible for a liquid and a solid 
body to occupy the same space at the same time, as it 
is for two solid bodies to do so. The reason why flu- 
ids appear less impenetrable than solid bodies, is, that 

18. When does matter exist in a solid form! 19. When in a fluid form; 
20. How many essential properties of matter are there .' What are they ; Why 
are they called essential properties; 21. What other properties exist in differ- 
ent bodies ? Why are they called accidental properties ; Are color and weight 
essential or accidental properties I Why ; 22. What terms are used in Philoso- 
phy to express the state in which matter exists ! 23. What is meant by Impene- 
trability I 24. Does Impenetrability belong to fluids ? Why do fluids appear 
less impenetrable than solid bodies ; 




8 NATURAL PHILOSOPHY. 

the particles of which they are composed move easily 
among themselves, on account of their slight degree of 
cohesion. [See No. 19.] 

Illustration 1st. Fill a tumbler with water, or any other liquid, 
and put a spoon, or any other article in it, — the liquid will flow 
over the sides of the vessel to make room for the spoon. 

Illustration 2d. Put some water into a tube closed at one end ; and 
then insert a piece of wood that fits the inside of the tube very ac- 
curately. It will be impossible to force the wood to the bottom of the 
tube, unless the water is first removed. The same experiment may 
be made with air instead of water ; and proves that water, air, and 
all other fluids, are equally solid, or impenetrable, with the hardest 
bodies. 

25. The particles of fluids are supposed to be round, 
F i lm r\ ari d therefore touch one another only 

in a few points. There will be spaces 
between the particles of fluids, in the 
,same manner that there are between 
large balls which are piled on one an- 
other. Between these spaces other 
smaller balls may be placed ; and these smaller 
balls, having spaces between them, will admit others 
still smaller ; as may be seen in Fig. 1. 

It may thus be perceived, that all substances, whose particles are 
globular, or round, have vacant spaces between the particles, which 
can never be filled. For this reason, a certain quantity of salt, the 
particles of which are smaller than those of water, can be put into a 
vessel full of water, without causing it to overflow; and as the par- 
ticles of which sugar is composed are smaller than those of salt, a 
portion of sugar may be added after the fluid is saturated with salt. 

26. The impenetrability of water was shown by an 
experiment made at Florence, many years ago. A 
hollow globe of gold was filled with water, and submit- 
ted to great pressure. The water was seen to ex- 
ude through the pores of the gold and covered it with a 
fine dew. [See note under No. 196.] 

27. When an open phial is plunged into a basin of 
water, the air will rush out in bubbles to make room for 
the water ; and if an inverted tumbler or goblet be im- 

What examples are given in Illustration 1st and 2d to prove the impenetrabil- 
ity of fluids ! 25. What is supposed to be the form of the particles of fluids; 
What follows from this ! What figure illustrates this ! Whut is said in regard 
to all bodies whose particles are round or globular.' What examples are given 
to prove this? 26. What example can you give to prove the impenetrability of 
water I 27. What the air I 



MATTER AND ITS PROPERTIES. M 

mersed in water, the water will not rise in the tumbler 
unless it be inclined so that the air can escape. These 
are further proofs of the impenetrability of air. 

28. When a nail is driven into wood, or any other 
substance, it forces the particles asunder, and makes 
its way between them ; but not a single atom of the 
wood can remain in the same space that the nail occu- 
pies ; and if the wood is not increased in size by the 
addition ofthe nail, it is because wood is a porous sub- 
stance, like sponge, the particles of which may be 
compressed or squeezed more closely together. It is 
thus they make way for the nail. 

29. By extension, is meant length, breadth and 
depth. Bulk and size are but different names for ex- 
tension. It is evident that every body, or portion of 
matter must have size, bulk, or extension, which is 
measured by the portion of space which it occupies. 

30. The different terms which are used to express 
the extension of a body are length, breadth, width, 
height, depth and thickness. Length is the extent from 
end to end. Breadth or width is the extent from side 
to side. Height, depth and thickness, are the extent 
from the top to the bottom. The measure of a body 
from the bottom to the top is called height, from the 
top to the bottom is called depth. Thus we speak of 
the depth of a well, the height of a house, &c. Thick- 
ness is a term applied to solid bodies only, and implies 
the extent from the upper to the under surface. 

31. By figure, is meant the form or shape of a body ; 
and it differs from extension, in the quantity of matter 
it contains. Thus two circles, or two balls may be of 
the same shape or figure, while they differ in extension 
as the one exceeds the other in the quantity of matter 
which it contains, and consequently will occupy more 
space. 

32. By Divisibility, is meant susceptibility of being 
divided. A body, however small, can be divided into 
halves, quarters, &c. ; and these halves and quarters 

28. What solids ? 29. What is meant by Extension .' 30. What terms are 
used to express the extension of a body I What is length I Breadth .' Height, 
depth and thickness ! What is the difference between height and depth > What 
is thickness .' 31. What is meant by figure .' How does it differ from extension > 
Give an example to show the difference. 32. What is meant by Divisibility I 
2* 



10 NATURAL PHILOSOPHY. 

may be again divided in the same manner, although 
they may be too small to be visible to our eyes. There 
are some living creatures called animalcula,so small that 
we cannot see them. To them a grain of sand appears as 
large as a mountain does to us. Our power of divid- 
ing matter ends where theirs begins; and it follows 
that this divisibility of matter is limited only by the ex- 
tent of our powers. If, therefore, by means of cutting, 
pounding, grinding, &c, we divide a body into as 
small particles as we can, these particles will still have 
an upper and an under surface, with length, breadth, 
and thickness, all of which will still be visible to such 
creatures as have sufficient powers of vision. 

33. The extreme divisibility of matter may be shown 
in a number of ways. First. By melting a solid body 
in a liquid. When, for instance, we sweeten a cup of 
tea or coffee, a small portion of sugar is dissolved and 
diffused through the whole of the liquid. 

Secondly. From the manner in which we smell odo- 
riferous substances. The perfume or odor of a body 
is produced by the escape of very minute particles 
which enter the nostrils. This perfume is diffused 
through the whole extent of a large room, without the 
loss of the smallest visible part of the substance. 

Thirdly. A few drops of a colored liquid falling 
into a vessel of water, immediately tinges the whole of 
the water with the color, and must, therefore, be dif- 
fused throughout it. 

Fourthly. A lighted candle, placed upon a hill, dif- 
fuses particles of light through the space of a mile in 
extent, before it has lost any visible portion of its sub- 
stance. 

Fifthly. It has been calculated that sixteen ounces 
of gold, which in the form of a cube would not meas- 
ure an inch and a quarter in its side, will completely 
gild a quantity of silver wire twenty-five thousand miles 
in length. 

Sixthly. A single grain of gold may be hammered 
by a gold-beater until it will cover fifty square inches ; 

By what is the divisibility of matter limited '. 33. Mention some example to 
show the extreme divisibility of matter. What do these examples prove .' 



MATTER AND ITS PROPERTIES; J 1 

each square inch may be divided into two hundred 
strips; and each strip into two hundred parts, which 
may be seen with the naked eye. Each square inch, 
therefore, contains forty thousand visible parts, which, 
multiplied by fifty, the number of square inches which 
a grain of gold will make, gives two million parts, each 
of which can be seen with the naked eye. From all 
which it appears that matter is infinitely divisible. 

34. The particles which escape from luminous or 
odoriferous objects, although they are too small to be 
visible, all form a part of the substance of those ob- 
jects, and a body is in reality diminished by their es- 
cape. This is evident in liquid bodies; as, for instance, 
in a bottle of lavender water, which, if left unstopped 
a sufficient length of time, will evaporate and disap- 
pear. 

35. The steam which arises from boiling water is 
nothing more than portions of the water heated. The 
heat insinuates itself between the particles of the wa- 
ter and forces them asunder. When deprived of the 
heat, the particles will unite together in the form of 
drops of water. 

Experiment. Hold a cold plate over boiling water. The steam 
arising from the water, will unite in drops on the bottom of the 
plate. 

36. The air which we breathe generally contains a 
considerable portion of moisture or water. On a cold 
day this moisture condenses on the glass in the win- 
dows, and becomes visible. We see it, also, collected 
in drops on the ouside of a tumbler or other vessel 
containing cold water, in warm weather. 

37. By the Indestructibility of matter is meant that 
it cannot be destroyed. It may be indefinitely divided, 
or altered in its form, color and accidental properties, 
but it must still continue to exist in some form through 
all its changes of external appearance. 

34. Are odoriferous and luminous bodies diminished by the particles which escape 
from them I Why can we not see the particles which escape ! Give an example 
to prove that the bodies are diminished. 35. Of what is the steam, which arises 
from boiling water, composed ? How are the particles separated >. When de- 
prived of heat what will become of them .' What experiment is given to prove 
this >. 36. Does the air which we breathe contain any moisture .' Give an ex- 
ample to prove it. 37. V\ hat is meant by the indestructibility of matter ! 



12 NATURAL PHILOSOPHY. 

38. The science of chemistry teaches us that there is 
a certain defiuite number of elementary substances, of 
some one or more of which all other substances are 
composed. The powers of man, or of nature, can 
change the shape, the combination, or the situation of 
these elementary substances, but nothing short of crea- 
tive power can annihilate any one of them. 

Illustration 1st. Thus water, for instance, which was formerly 
considered as a simple substance, is found to consist of two substan- 
ces, almost imperceptible to the sight, called hydrogen and oxygen, 
united by what is called chemical attraction. These substances 
may be separated and made to unite w ith other substances, but they 
cannot be destroyed. 

Illustration 2d. There is actually no more nor less water exist- 
ing at the present time than there was at the creation of the world, 
but it exists only in different forms or situations. When water dis- 
appears, either by boiling over a fire, or evaporating by the heat of 
the sun, or, in other words, when " it dries up," it rises slowly in the 
form of steam or vapor. This vapor ascends in the air and con- 
stitutes clouds ; these clouds again fall to the earth in the shape of 
rain, snow or hail, and form springs, fountains, rivers, &c. The 
water on or in the earth, therefore, is constantly changing its shape 
or situation, but no particle of it is ever actually destroyed. 

Illustration 3d. The particles or simple substances of which wood 
or coal is composed, are not destroyed when the wood or coal is 
burnt. Part of them arise in smoke or vapor and the remainder is 
reduced to ashes. A body in burning undergoes remarkable chang- 
es. It is subdivided — its form and color are altered— its extension 
is increased, but the various parts into which it has been separated 
by combustion, continue in existence and retain all the essential 
properties of bodies. 

Illustration 4th. Every thing in nature decays and is corrupted in 
the lapse of time. We ourselves die, and our bodies moulder in the 
dust; but not a single atom of them is lost. They serve to nour- 
ish the earth, whence, while living, they drew their support, and by 
degrees become incorporated with other substances. 

39. By Inertia, is meant the resistance which inac- 
tive matter makes to a change of state, whether of mo- 
tion or rest. A body at rest cannot put itself in mo- 
tion, nor can a body in motion stop itself. 

40. A body, when put in motion, will continue to move 

38. What does chemistry teach with regard to the composition of bodies > Can 
any particle of matter be annihilated; Of what is water composed >. Is there 
more or less water existing now than there was at the creation of the worJd >. 
What becomes of water when it evaporates > What becomes of the particles or 
simple substances of different kinds of fuel when burnt .' What becomes of ev- 
ery thing in nature .' 39. What is meant by Inertia I 40. How long will a body 
in motion continue to move, unless it be stopped .' 



MATTER AND ITS PROPERTIES. 13 

forever, unless it be stopped. When a stone or ball is 
thrown from the hand, there are two forces which con- 
tinually operate to stop it ; viz. the resistance of the air, 
and gravitation : all motion which is caused by animal or 
mechanical power, will be destroyed by the combined 
action of these forces. But could these obstacles be re- 
moved, the body in motion would continue to move 
forever. 

41. The Inertia, or resistance of a body to a change 
of state, as, for instance, a ball, may be perceived by 
throwing it from the hand. It requires a considerable 
degree of strength to give it a rapid motion ; and the 
person who stops or catches it, feels the resistance it 
makes to being stopped. 

42. The degree of motion in a moving body, or the 
force which it will require to stop it when in motion is 
called its momentum, and is calculated by its velocity, 
its size, and its weight, or the quantity of matter which 
it contains. The smaller its size,* and the greater its 
weight and its velocity, the greater will be its momen- 
tum. 

Illustration. Thus, if a body weighing six pounds, move at the 
rate of two miles in a second of time, its momentum maybe repre- 
sented by six, multiplied by two, which is equal to twelve. If a 
body weighing twelve pounds, move at the rate of four miles in 
the same time, its momentum will be represented by twelve, multi- 
plied by four, which is equal to forty-eight. 

43. Attraction expresses the tendency which differ- 
ent bodies or portions of matter have to approach each 
other. Every portion of matter is attracted by every 
other portion of matter, and this attraction is the 
strongest in the largest portions. 

* The resistance of the air will be less on a small body than on a 
large body. Could this resistance be removed, the momentum of a 
body would depend only on its velocity and weight. 

When a stone or ball is thrown from the hand how many forces continually ope- 
rate to stop it .' What are they .' What destroys the motion caused by animal 
or mechanical power >. How could a body in motion be made to move forever '. 
41. What example is given to show the Inertia of a body .' 42. What is the mo- 
mentum of a body ! Flow is it calculated ; Upon what does the momentum of a 
body depend '. Is the resistance of the air less on a large or small body >. If this 
resistance could be removed, upon what would the momentum depend .' What il- 
lustration is given! 43. What is attraction ? Where is attraction the strong- 
est I 



14 NATURAL PHILOSOPHY. 

44. As the earth is the largest portion of matter with 
which we are practically acquainted, every thing on or 
near its surface, is attracted towards it. For this rea- 
son every thing about us will fall to the ground or the 
surface of the earth, unless it is prevented. 

45. The attraction of all masses of matter is in a 
direct proportion to their quantity, and in inverse pro- 
portion to their distances from each other. That is, 
the greater the quantity and the less the distance, the 
stronger will be the attraction. 

46. There are two kinds of attraction belonging to 
all matter, namely, the attraction of gravitation, or 
gravity:, and the attraction of cohesion, or cohesive 

ATTRACTION. 

47. The attraction of gravitation, or gravity, is that 
which causesbodiesat a distance to approahc each other. 

48. The attraction of cohesion, or cohesive attrac- 
tion, is that which unites the particles of a body. [See 
No. 16.] 

49. By the attraction of gravity a stone falls to the 
ground. 

50. By the attraction of cohesion, the particles which 
compose the stone are held together. 

51. The difference between the two kinds of attrac- 
tion is this: the attraction of cohesion takes place in 
very minute particles, and at very small distances ; the 
attraction of gravity acts on the largest bodies and at 
immense distances. The attraction of cohesion takes 
place between the particles of the same body. The 
attraction of gravitation causes different bodies to ap- 
proach each other. 

52. The attraction of gravitation causes weight ; or, 
in other words, weight is but another name for attrac- 
tion. When we say that a body weighs an ounce, a 
pound, or a hundred pounds, we express by these 
terms, the degree of attraction by which it is drawn to- 

44. Wliy is every thing attracted towards the earth.' 45. In what proportion 
does attraction inciease! 46. How many kinds of attraction are there belong- 
ing to all matter .' What are they .' 47. What is the attraction of gravitation, 
or gravity .' 48. What is the attraction of cohesion, or cohesive attraction .' 49. 
What causes a stone to fall to the ground .' 50. By what aro the particles which 
compose the stone held together .' 51. What is the difference between these at- 
tractions ! 52. What is weight .' When we say a body weighs an ounce, or a 
pound, what do we express by this term.' 



MATTER AND ITS PROPERTIES. 15 

wards the earth. As this attraction, (as was stated in 
number 43,) depends upon the quantity or portion of 
matter there is in a body, it follows that those bodies 
which are heaviest, that is, which are most strongly at- 
tracted, contain the most matter. 

53. We estimate the quantity of matter in a body, 
not by its apparent size but by its weight. Some bod- 
ies, as cork, feathers, <fcc, are light; others, as lead, 
gold, mercury, &c. are heavy. The reason of this is 
that the particles which compose the former are not 
closely packed together, and therefore they occupy 
considerable space ; while in the latter they are joined 
more closely together, and occupy but little room. A 
pound of cork and a pound of lead, therefore, will dif- 
fer very much in apparent size, while they are both 
equally attracted by gravity, that is, they weigh the 
same. 

54. The particles of which bodies are composed 
touch one another in few places only. There are, con- 
sequently, small spaces between the particles, and 
these spaces are called pores. The porosity of a body 
implies, therefore, that it has pores ; and the greater 
the number, and the larger the size of these pores, the 
more porous the body is said to be. 

55. The porosity of bodies leads to another distinc- 
tion, called density and rarity. By density is meant 
the closeness and compactness of the particles of a 
body. Rarity is the contrary of density, and implies 
the thinness or subtlety of bodies. A body in which 
the pores are small and few in number is called a dense 
body. When the pores are large and numerous, the 
body is said to be rare. 

56. Dense bodies are always heavier than rare bod- 
ies of the same size, because there are a greater num- 
ber of particles in the same space, and consequently 
the body is more strongly attracted. [See No. 53.] 

Upon what does attraction depend ? What follows from this ? 53. How do 
we estimate the quantity of matter in a body ? What bodies are light I What 
heavy ? How do you account for this difference ? 54. What are pores ? What 
does the porosity of a body imply > Upon what does the porosity depend? 55. 
What is meant by Density > Rarity > When is a body called dense ? When rare >. 
56. How do dense and rare bodies of the same size compare with regard to their 
weight? Why J 



16 NATURAL PHILOSOPHY. 

57. The strength of cohesive attraction in bodies, 
depends, in great measure, upon their density. This 
is particularly the case in fluids. The thinner and 
lighter a fluid is, the less is its cohesive attraction. 

58. The cohesive attraction of solid bodies is much 
greater than that of fluids ; because the particles of 
solids are more closely united. 

59. The pores of substances are generally filled 
with air. 

60. There is another fluid much more subtle than 
air, which pervades all bodies ; namely, heat, which 
insinuates itself, more or less, between the particles of 
all bodies, and forces them asunder. Heat, and the 
attraction of cohesion constantly act in opposition to 
each other ; and the more a body is heated, the more 
its particles will be separated. 

61. The eflfect of heat in separating the particles of 
different kinds of substances is seen in the melting of 
solids, such as metals, wax, butter, &c. The heat in- 
sinuates itself between the particles, and forces them 
asunder. These particles then are removed from that 
degree of nearness or proximity to each other within 
which cohesive attraction exists, and the body is re- 
duced to a fluid form. When the heat is removed the 
bodies return to their former solid state. 

62. From what has now been stated, it appears, 1st. 
That cohesive attraction exists only between those par- 
ticles which are, or appear to be, in absolute contact. 
2d. That cohesive attraction is the strongest when the 
particles are in a state of closest contact, 3d. That 
the cause of the fluid form of bodies, is, that the par- 
icles touch one another only in a few points, and there- 
fore that cohesive attraction in them is weak. [See 
No. 19.] 

57. Upon what does cohesive attraction in a great measure depend? In what 
is this particularly the case ; Upon what does ihe cohesive attraction of fluids 
depend? 58. In which is cohesive attraction the stronger, in solids or fluids.' 
Why.' 59. Wirh what are the pores of all substances generally filled ? 60. What 
fluid is more subtle than air? What effect has heat upon bodies! What two 
forces continually act in opposition to each other .' 61. In what can the effect of 
heat be seen; How does it separate the particles' What would be the effect 
were the heat removed ? 62. From what has been stated, what do you learn res- 
pecting cohesive attraction ! First; Second? Third; 



MATTER AND ITS PROPERTIES. 17 

63. Of all the effects of heat, that produced upon 
water, is, perhaps, the most remarkable. The particles 
are totally separated and converted into steam or va- 
por, and their extension is wonderfullyincreased. [See 
No. 35.] 

64. Heat also produces most remarkable effects upon 
air, causing it to expand to a wonderful extent, while 
the absence of heat causes it to shrink or contract into 
very small dimensions. 

65. The attraction of cohesion causes the small wa- 
tery particles which compose mist or vapor to unite 
together in the form of drops of water. It is thus that 
rain is produced. The clouds consist of mist or vapor 
expanded by heat. They rise to the cold regions of 
the skies, where they lose their heat; and then, uniting 
in drops, fall to the earth. But so long as they retain 
their heat, the attraction of cohesion can have no in- 
fluence upon them, and they will continue to exist in 
the form of steam, vapor, or mist. 

The attraction of cohesion also causes liquids to rise above their 
level in capillary tubes, or tubes the bores of which are exceedingly 
small. This is caused by the attraction between the particles of the 
liquid and the interior surface of the tube. 

Experiment. The same effect is produced by two pieces of flat 
glass, joined together at one side, and separated at the other by a 
thin strip of wood, card, or other substance. When thus prepared, 
immerse the glass in colored water, having previously wet the in- 
ner surfaces. The water will rise between the pieces of glass, 
forming a beautiful curve, the highest part appearing where the 
pieces of glass are in contact. 

66. All porous substances, such as sponge, bread, 
linen, sugar, &c, may be considered as collections of 
capillary tubes, and for this reason, water and other 
liquids will rise in them, when they are partly im- 
mersed. 

67. By the Compressibility of bodies is meant the 
power of being compressed into smaller limits of ex- 

63. Upon what has heat the most remarkable effect .' How does it effect it ! 
64. What effect has heat upon air ? 65. How is rain produced >. Of what do the 
clouds consist ? What, causes liquids to rise above their level in capillary tubes ? 
How does it cause them to rise I What experiment is given to show the effect of 
capillary aitraction ; 66. What is the reason that liquids will rise in porous sub- 
stances when only partially immersed in them .' 67. What is meant by the com- 
pressibility of bodies ? 



18 NATURAL PHILOSOPHY. 

tension than they naturally have. Of this all sub- 
stances are susceptible if a sufficient power be applied. 

68. By the Expansibility of bodies, is meant the 
power of being increased in extension, and it is the 
reverse of Compressibility. Heat expands most sub- 
stances, and cold contracts or compresses them. 

69. By Mobility is meant the power of being moved. 
All bodies however large or heavy, may be moved, pro- 
vided a sufficient power be applied. 

70. Elasticity is the power which causes a body to 
resume its shape after being compressed or expanded. 
Thus when a bow or a cane is bent, it returns to its 
former shape as soon as the pressure is removed. 
When the flesh of a living animal is pressed, it in like 
manner resumes its shape on removing the pressure. 
Caoutchouc, or India-rubber likewise, on account of 
its elasticity, when bent or drawn out, will immediately 
return to its shape. But of all bodies, those in the 
form of gas, or air, are the most remarkable for this 
property. Hard bodies* are in the next degree elas- 

* Whoa two ivory or metallic balls strike each other, the partd at which they 
touch will be flattened, but no mark is perceptible, their elasticity instantly de- 
stroying all trace of it. If, however, a small spot of ink be placed on one of the 
balls at the point of contact, it will be found, after the contact, to have spread, 
and will thus show that there has been compression. The cause of elasticity is 
not well understood. Elasticity implies susceptibility of compression ; and the 
susceptibility of compression depends upon the porosity of bodies : for were there 
no pores, or spaces between the particles of matter, of which a body is composed, 
it could not be compressed. But it is not the case that bodies whose particles 
are most distant from each other are the most elastic. Elasticity implies not only 
susceptibility of compression, but the power of restoring its former state after 
compression. The pores of such bodies as ivory and metals, are invisible to the 
naked eye; but it is well ascertained that gold, [See No. 26.] one of tho most dense 
of all bodies is extremely porous, and that its pores are sufficiently large to ad- 
mit water, under great pressure, to pass through them. In cork, sponge, and bread, 
the poies form considerable cavities •, in wood, and many kinds of stone, when not 
polished, they are perceptible to the n^ked eye; whilst in ivory, metals, and most 
varnished and polished bodies they cannot be discerned. To give an idea of the 
extreme porosity of bodies, Sir Isaac Newton conjectuted, that if the earth were so 
compressed as to be absolutely without pores, its dimensions might not be more 
than a cubic inch. The elasticity of ivory is very perfect, that is to say, it re- 
stores itself after compression, with a force very nearly equal to that exerted in 
compressing it. Liquids, such as water, etc. have scarcely any elasticity. 

Can all bodies be compressed ? 68. What is Expansibility I What effect has heat 
and cold upon bodies ! 69. What is Mobility >. 70. What is Elasticity.' Give some ex- 
amples of Plasticity. What bodies are most elastic I Which are the moreelastic, 
hard or soft bodies? Note. Whateffctis produced when two ivory balls strike each 
other; What is the cause of Elasticity .' Could a body without pores be compressed 
Are those bodies most clastic whose particles are most, distant? What does Elas-? 
ticrty imply .' How has it been proved that gold is porous ? [See No. 26.] What 
is said of the pores of cork, sponge, wood, stone, ivory, metals, &c. ? What did 
Newton conjecture with regard to the pores of the earth? Which has the most 
elasticity, ivory or liquids ? 



WEIGHT, OR GRAVITATION. 19 

tic. Soft bodies, such as clay, wax, tallow, butter, &c. 
have very little elasticity, and are called non-elastic 
bodies. 

Malleability implies capability of being drawn under the hammer, 
or rolling-press. This property belongs to some of the metals, as 
gold, silver, iron, copper, &c, but not to all; and it is of vast impor- 
tance to the arts and conveniences of life. Gold is the most mallea- 
ble of all metals. 

Brittleness is the property which renders substances easily brok- 
en, or separated into irregular fragments. This property belongs 
chiefly to hard bodies. 

Iron, steel, brass and copper become brittle when heated and sud- 
denly cooled; but if cooled slowly, they are not easily broken. 
Brittleness is not entirely opposed to elasticity ; for some bodies, 
glass, for instance, [are very brittle, and yet a ball, or fine threads 
of this substance, are highly elastic. 

Brittleness is the opposite of malleability. 

Ductility is that property which renders a substance susceptible of 
being drawn into wire. Platina is the most ductile of all metals. 
It can be drawn into wire scarcely larger than a spider's web. 
Malleability and ductility are different properties. Some substan- 
ces which are malleable to a great degree have very little ductility ; 
and on the contrary, some which are very ductile are not malle- 
able. 

Tenacity implies a great degree of cohesion among the particles 
of bodies. Steel is the most tenacious of all substances. 



SECTION III. 

Of Gravity or Weight, or the Attraction of Gravitation. 

71. All matter is attractive, from the smallest parti- 
cle to the largest mass ; and bodies attract each other 
with a force proportionate to their density, that is, the 
quantity of matter they contain. 

72. The earth being the largest mass of matter with 
which we are practically acquainted, attracts {accord- 
ing to the principles stated in numbers 43, 44 and 45) all 
things to itself. This attraction is called the attraction 
of gravitation. 

71. Is all matter attractive ? In what proportion do bodies attract each other ? 
72. What is sajd of the attraction of the earth, and what is the attraction called > 



20 NATURAL PHILOSOPHY. 

73. The force of gravity is greatest at the surface of 
the earth, and decreases both upwards and downwards, 
but in different degrees. 

74. The force of gravity decreases above the sur- 
face as the square of the distance from the centre in- 
creases. That is, gravity at the surface of the earth, 
which is about 4000 miles from the centre, is four times 
more powerful than it would be at double that dis- 
tance, or 8000 miles from the centre. 

75. According to the principle just stated, a body 
which at the surface of the earth weighs a pound, at 
the centre of the earth will weigh nothing. 

1003 miles from the centre it will weigh one quarter of a pound 
2000 " " " " " " one half of a pound. 
3000 " " " " " " three quarters of a pound. 
4000 " " " " " " one pound. 
8000 " " " " " " one quarter. 
12000 « " " " " " one ninth, &c. 

76. It follows from what has been stated, with regard 
to weight as a consequence of attraction, that if there 
were but one body in the universe, it would have no 
weight, because there would be nothing to attract it. 
But cohesive attraction would still exist, and keep the 
particles which compose the body united. 

77. As the attraction betw r een all bodies is mutual, 
it follows that when a stone or any heavy body falls to 
the earth, the earth will rise to meet it. But as the 
attraction is in proportion to the quantity of matter 
each contains, the stone will fall as much farther than 
the earth rises, as the earth exceeds the stone in bulk. 
Now the earth is one quatrillion, that is, one thousand 
million millions times larger than the largest body 
which has ever been known to fall through our atmos- 
phere. Supposing, then, that such a body should fall 
through a distance of 1000 feet — the earth would rise 
no more than the hundred billionth part of an inch, a 
distance altogether imperceptible to our senses. 

73. Where is the force of gravity greatest! 74. In what proportion does grav- 
ity decrease above the surface of ihe earth! 75. Give an example to show this.- 
76. If there were but one body in the universe, what would be its weight ? Why > 
Would cohesive attraction exist? 77. Is the attraction between bodies mutual > 
What follows from this > Why do we not see the earth rise to meet falling bod- 
ies ? What example is given to illustrate this I 




WEIGHT, OR GRAVITATION. 21 

78. The principle of mutual attraction, is not con- 
fined to the earth. It extends to the sun, the planets, 
comets and stars. The earth attracts each of them, 
and each of them attracts the earth, and these mutual 
attractions are so nicely balanced by the power of 
God, as to cause the regular motions of all the heavenly 
bodies, the diversity of the seasons, the succession of 
day and night, summer and winter, and all the grand 
operations which are described in astronomy. 

79. The direction in which falling bodies approach 
the surface of the earth, is called a vertical, or plumb 
line. Such lines are every where perpendicular to the 

No. 2. surface, and when prolonged will meet 
at the centre of the earth. For this rea- 
son no two lines suspended by weights 
will be parallel to each other. Even a 
pair of scales hanging perpendicular to 
the earth, are not exactly parallel, be- 
cause they both point to the same spot, 
namely, the centre of the earth — but the 
convergency is so small, that their inclination is not 
perceptible to our senses. [See Fig. 2.] For the same 
reason no two bodies can fall to the earth in parallel 
lines. 

80. According to the laws of attraction, all bodies 
at an equal distance from the earth will fall to it in the 
same space of time, if nothing impedes them. But the 
resistance of the air makes bodies of different density 
fall with different degrees of velocity ; and as dense, 
hen\v bodies, by their greater momentum (See number 
42) overcome this resistance more easily than rarer or 
lighter ones, they will fall with greater velocity. 

81. The resistance which the air opposes to the fall 
of bodies is proportioned to their surface, not to their 

78. What is said of the extent of the power of mutual attraction ? 79. What is a 
vertical or plumb line ? How are these lines situated with regard to the earth's 
surface? Where will these lines meet, if prolonged .' Why are not two lines 
suspended by weights parallel ? Are not a pair of scales, hanging perpendicular to 
thee. .iui, y .1 hli vVhy ? Why do they appear parallel? Can any two bod- 
ies fall to the earth in parallel lines > 80. What is the reason that all bodies, at 
equal distances from the earth, do not fall in the same space of time; What 
bodies fall with the greatest velocity I 81. To what is the resistance of the air, 
in falling bodies, proportioned > 

3* 



22 



NATURAL PHILOSOPHY. 



weight. Heavy bodies can be made to float in the air, 
instead of falling immediately to the ground, by making 
the extent of their surface counterbalance their weight. 
Thus gold, which is one of the heaviest of all sub- 
stances, when spread out into thin leaf, is not attracted 
by gravity with sufficient force, to overcome the resist- 
ance of the air ; it therefore floats, as it were, in the 
air, and falls slowly. 

82. All substances of whatever nature are equally 
influenced by gravity, in exact proportion to their den- 
sity. Even air itself, light as it seems, is subject to 
this attraction. The air extends to a height of more 
than forty-five miles above the surface of the earth. 
The pressure of the upper parts of the atmosphere on 
those beneath, renders the air near the surface of the 
earth much more dense than that in the upper regions. 
This pressure is caused by the attraction of the earth, 
or the weight of the air above ; and it would cause the air 
to fall like other bodies completely to the earth ; were 
it not for the elasticity (See No. 70,) of that portion 
which is near the surface. The air therefore of which 
the atmosphere is composed, exists in a state of con- 
stant compression, and is heavier near the surface of 
the earth, and grows lighter as we ascend. Gravity 
brings the particles together, while elasticity gives them 
a constant tendency to expand. Gravity thus confines 
the air to the region of the earth, while elasticity pre- 
vents it from falling like other bodies to the ground. 

83. Smoke, steam, and all similar substances, are 
affected by gravity in a similar manner. But as 
gravity acts on all substances in exact proportion 
to their density, and the air near the surface of 
the earth is more dense than smoke, steam, &c, the 
air and other similar fluids will arrange themselves in 
obedience to this law, in regular order, according to 
their different densities. Accordingly as smoke, steam, 
&c, are less dense than the air near the surface of the 

How can heavy bodies be made to float in the air I Give an example to illus- 
trate this. 82. In what proportion arc all substances influenced by gravity! Is air 
affected by it ! How far does the air extend above the surface of the earth ? What 
causes the air to be more dense at the surface of the earth i What causes this 
pressure >. Why does not the air fall to the earth like other bodies? 38. What caus- 
es steam and other similar substances to rise' In what proportion does gravity 
act upon bodies ? 



WEIGHT, OR GRAVITATION. 23 

earth, they will rise until they reach a portion of the 
air, of the same density with themselves, where they 
will remain stationary. 

84. The specific gravity of bodies is a term used to 
express the relative weight of equal portions of differ- 
ent bodies. We know that a portion of lead will 
weigh more than a portion of wood of the same size. 
A piece of wood will weigh more than a piece of cork 
of the same dimensions, and cork will weigh more than 
a portion of air, smoke or vapor of the same extension. 
Hence we say that the specific gravity of cork is great- 
er than that of air, the specific gravity of wood is great- 
er than that of cork, and the specific gravity of lead 
greater than that of wood, &c. 

85. From what has now been said with respect to 
the attraction of gravitation and the specific gravity 
of bodies, it appears that although the earth attracts all 
substances, yet this very attraction causes some bodies 
to rise and others to fall. Those bodies or substances 
the specific gravity of which is greater than that of air 
will fall, and those whose specific gravity is less than 
that of air will rise, — or rather, the air being more 
strongly attracted will get beneath them, and, thus dis- 
placing them, will cause them to rise. For the same 
reason, cork and other light substanceswill not sink in 
water, because the specific gravity of water being 
greater, the water is more strongly attracted and will 
be drawn down beneath them. [For a table of the spe- 
cific gravity of bodies, seethe note under No. 213.] 

86. The principle which causes balloons to rise, is 
the same which causes the ascent of smoke, steam, &c. 
The materials of which a balloon is made, are heavier 
than air, but their extension is greatly increased, and 
they are filled with an elastic fluid of a different nature, 
specifically lighter than air, so that on the whole 
the balloon when thus filled is much lighter than a 
portion of air of the same size or dimensions, and 
it will consequently rise. 

Wlien will smoke, steam, and other similar substances remain stationary ? 84. 
What is specific gravity > Illustrate this. 85. Does the attraction of the earth 
cause all bodies lu fall ? What bodies will fall ? What rise? How does the air 
cause them to rise >. W T hy do not cork and other light bodies sink in water ? 
Explain the principle upon which balloons rise. 



24 NATURAL PHILOSOPHY. 

87. Gravity, therefore, causes bodies which are light- 
er than air to ascend, those which are of equal weight 
with air to remain stationary, and those which are 
heavier than air to descend ; but the rapidity of their 
descent is affected by the resistance of the air ; which 
resistance is proportioned to the extent of the surface 
of the falling body. 

88. From what is stated in number 80, it appears 
that as a dense body, such for instance as a piece of 
metal or money, is more strongly attracted by the earth 
than a rarer (that is to say, a lighter) one, its momen- 
tum will enable it to overcome the resistance of the air 
more readily, and that it will consequently fall to the 
ground more quickly than a lighter one. But if the 
resistance of the air could be removed, they would 
both fall in precisely the same time. [This will be il- 
lustrated by experiment in connexion with pneumatics.] 
89. It has been stated (SeeNo. 73) that the force of gravity 
is the greatest at the surface of the earth, and decreases 
both upwards and downwards, but in different degrees. 
But the diminution of its force at so small a distance 
as that to which the atmosphere extends is so incon- 
siderable, compared with the size of the earth, as to be 
scarcely perceptible. The greatest height ever attained 
by man in balloons or on the summit of the highest 
mountains scarcely exceeds a thousandth part of the 
distance from the centre to the surface of the earth. 
Although, therefore, it is true that the air near the sur- 
face of the earth is more strongly attracted than that 
in the upper regions of the atmosphere, yet the differ- 
ence is so exceedingly small, that it is imperceptible. 
But the weight of the upper air resting upon the lower 
(as is stated in No. 82,) compresses it into smaller vol- 
ume and thereby increases its density. This increase 
of its density causes a corresponding increase of its 
gravity (according to the principle stated in No., 71.) 

87. What effect has gravity on bodies lighter than the air? What effect on 
bodies of equal weight ? What effect on those that are heavier? What affects 
the rapidity of their descent .' To what is the resistance of the air proportioned ? 
88. Which is more strongly attracted by the earth, a dense or a rare body ? What 
follows from this: How can they be made to fall at the same time; 89. Where 
is gravity the greatest .' Why is not the diminution of it, as we go from the sur- 
face of the earth, very apparent? What is the greatest height in the air ever 
attained byman ? 



MECHANICS. 25 

90. The pressure of the atmosphere has been com- 
pared to that of a pile of fleeces of wool, in which the 
lower fleeces are pressed together by the weight of 
those above. The uppermost fleece, receiving no ex- 
ternal pressure, is confined merely by the force of 
its own gravity. 

91. Smoke consists of minute particles of fuel car- 
ried up by a current of heated air from the fire below. 
As heat expands all bodies [See No. 60,] it conse- 
quently rarefies (that is, expands) air, and renders it 
lighter than the colder air of the atmosphere. The 
heated air from the fire, carries up with it vapor 
and small particles of the combustible materials which 
are burning in the fire. When this current of hot air 
is cooled by mixing with the atmosphere, the mi- 
nute particles of coal or other combustibles fall, and 
it is this which produces the small black flakes which 
frequently render the air, and every thing in contact 
with it so dirty. This is particularly the case in large 
cities, where bituminous coal is used for fuel. 

92. From what has now been stated, it appears that 
gravitation is the force which occasions the fall of 
bodies ; cohesion, that which binds the particles of 
bodies together ; and heat a force which drives them 
asunder. These three powers may be comprehended 
under two names, Attraction and Repulsion. 



SECTION IV. 

Mechanics, or the Laws of Motion. 

[For a definition of Mechanics, see No. 2.] 

93. A body is in motion whenever it is changing its 
situation with regard to a fixed point. Motion there- 
fore is a continued change of place. 

90. To what has the pressure of the atmosphere been compared >. 91. Of what 
does smoke consist ! What effect has heat upon bodies? What follows from 
this ? What produces the small black flakes which frequently float in the air i 
99. What is gravitation? Cohesion? Heat! Under what names may these 
powers be comprehended ? 93. What is motion .» 



26 NATURAL PHILOSOPHY. 

94. On account of the inertia [See No. 39,] of all 
matter, a body cannot put itself in motion, nor when it 
is in motion can it stop itself. The power which puts 
a body into motion is called a force, — and the power 
which has a tendency to stop or impede motion is 
called resistance. Thus, the stroke of a hammer is the 
force which drives a nail ; the pulling of the horse is 
the force which draws the carriage. Force, then, is the 
cause which produces motion. [See No. 93] 

95. The motion of a body impelled by a single force 
is always in a straight line, and in the same direction 
in which the force acts. 

96. The rapidity with which a body moves, or the 
length of time which it takes to move from one place 
to another, is called its velocity. 

97. The first law of motion is that the velocity of a 
moving body is always proportional to the force by 
which it is put in motion ; and that what is gained in 
power is lost in time. 

98. The velocity of a moving body is ascertained by 
the time that it occupies in passing through a given 
space. The greater the space, and the shorter the 
time, the greater is the velocity. Thus, if one body 
goes through six miles in an hour and another through 
twelve miles in the same time, the velocity of the latter 
is double that of the former. 

99. Velocity is sometimes called absolute, and some- 
times relative. Velocity is called absolute when the 
motion of a body in space is considered without ref- 
erence to that of other bodies, When, for instance, a 
horse goes a hundred miles in ten hours, his absolute 
velocity is ten miles an hour. 

100. Velocity is called relative when it is compared 
with that of another bedy. Thus, if one horse travels 
only fifty miles in ten hours, and another one hundred, 

94. Why cannot a body put itself in motion? Why cannot a body stop itself 
when in motion >. W T hat is force ! What is resistance ? What illustrations are 
given? 95. When is the motion of a body in a straight line? In what direction 
will it move ? 96. What is meant by velocity? 97. What is the first law of 
motion? 98. How is the velocity of a moving body ascertained? If one body 
goes through six miles in an hour, and another twelve, how does the velocity of 
the last compare with that of the first ? 99. What is meant by absolute velocity I 
Give an example. 100. When is the velocity of a body termed relaiive .' 



MECHANICS. 27 

in the same time, the absolute velocity of the first 
horse is five miles an hour, and that of the latter is ten 
miles ; but the relative velocity of the latter is double 
that of the former. 

10J. The velocity of a body is measured by the space 
over which it moves, divided by the time which it em- 
ploys in the motion. Thus, if a body moves one hundred 
miles in twenty hours, the velocity is one hundred di- 
vided by twenty, that is, five miles an hour. 

102. The time employed by a body in motion may 
be ascertained by dividing the space by the velocity. 
Thus, if the space be one hundred miles, and the veloci- 
ty five miles in an hour, the time will be one hundred 
divided by five, which is twenty hours. 

103. The space also may be ascertained by multiply- 
ing the velocity by the time. Thus if the velocity be 
five miles an hour, and the time twenty hours, the 
space will be twenty multiplied by five, which is one 
hundred miles. 

104. There are four terms applied to motion to ex- 
press its kind ; namely, uniform, accelerated, retarded, 
and perpetual motion. 

105. JJniform motion is that of a body passing over 
equal spaces in equal times. 

106. Accelerated motion is that in which the veloci- 
ty continually increases as the body moves. 

107. Retarded motion is that in which the velocity 
decreases as the body moves. 

108. Perpetual motion is that which continues either 
in a uniform, accelerated, or retarded state, without 
limit. 

109. Uniform motion is produced by a force having 
acted on a body and then ceasing to act. A ball 
struck by a bat, or a stone thrown from the hand, is, 
in theory, an instance of uniform motion ; but as the 
ball in both cases has to encounter the attraction of 

Give an example 101. How is the velocity of a body measured? Illustrate 
this. 102. How do you ascertaiu the time employed by a body in motion ! Illus- 
trate this. 103. How cai you ascertain the space; Illustrate this. J04. How 
many terms are applied to motion to express its kind I What arc they I 105. 
What is uniform motion.' 106. Accelerated? 107. Retarded? 108. What is 
perpetual motion ? 109. How is uniform motion produced I Why is not a oalJ 
struck by a bat, or a stone thrown Iroin the hand, an instance of uniform motion? 



28 NATURAL PHILOSOPHY. 

gravity on the one hand, which has a tendency to draw 
it to the ground, and the resistance of the air on the 
other, it in fact becomes an instance of retarded mo- 
tion. But if both the attraction of gravity and the re- 
sistance of the air could be entirely removed, it would 
proceed onwards in a straight line and with a uniform 
motion forever. 

110. Accelerated motion is produced by the contin- 
ued action of one or more forces. Thus, when a stone 
falls from a height, the impulse which it receives from 
gravity would be sufficient to bring it to the ground, 
with a uniform velocity; namely, sixteen feet every sec- 
ond of time. But the stone while falling at this rate is 
still acted upon by gravity with an additional force, 
which continues to impel it during the whole time of 
its descent. 

It is therefore found that during the first second it falls sixteen 
feet, three times that distance in the next, five times in the third, 
seven times in the fourth, and so on, regularly increasing its veloci- 
ty according to the number of seconds consumed in falling. The 
height of a building, or the depth of a well, may thus be measured 
by observing the length of time which a stone takes in falling from 
the top to the bottom. 

111. Retarded motion is produced when a body in 
motion encounters a force operating in an opposite di- 
rection from the motion. Thus when a stone is thrown 
perpendicularly upwards, the force of gravity is con- 
tinually operating in the opposite direction, and at- 
tracting it downwards to the earth. The stone moves 
upwards slower and slower, until the upward motion 
ceases, and the body returns with accelerated motion 
to the earth. It is found that when a body is thrown 
perpendicularly upwards, it takes the same length of 
time in ascending, that it takes in descending. 

112. Perpetual motion has never yet been produced by 
art, because gravity ultimately destroys all motion, that 
human powers can produce. But nature abounds with 

How can it be made an instance ? 110. How is accelerated motion produced >. 
Give an instance of accelerated motion. How far does a stone fall the first second 
of time? The second? Third! Fourth? How can you measure the height of 
a building, or the depth of a well ? 111. How is retarded motion produced ? Give 
an example. How does the time of the ascent and descent of a body thrown per- 
pendicularly, compare ? 112. Why cannot perpetual motion be produced ? 



MECHANICS. 29 

examples of perpetual motion, as for instance, the mo- 
tion of the heavenly bodies, described in the science of 
astronomy. 

113. The momentum of a body [SeeNo.42] is its degree 
of motion. In other words, the momentum of a body is 
the force or power with which a moving body would 
strike against another body. 

114. The momentum of a body may be ascertained, 
by multiplying* its weight by its velocity.! The 
quicker a body moves, the greater will be the force with 
which it will strike against another body ; £ so that 
a small, light body may have a greater momentum 
than a large heavy one, provided its velocity be suffi- 
ciently great. For instance, the momentum of an 
arrow, shot from a bow, is greater than that of a stone 
thrown from the hand, because its velocity is greater. 
But the momentum depends not alone on the velocity. 
On account of the inertia [See No. 39] of all matter, 
the greater the quantity of matter in a moving body, the 

* That the momentum is ascertained by multiplying, not by adding the weight 
and velocity, is proved by the following reasoning: If two bodies, one of one 
pound weight, the other of two pounds, have the same velocity, the moving force 
of the second, or its momentum, is double that of the first. If a third body, also 
of two pounds, move with three times the velocity of the second, its momentum, as 
the weights in this case are equal, is three times that of the second. But the 
momentum of the second is twice that of the first, therefore the momentum of the 
third is three times this quantity, or six times that of the first. By thus dividing 
the process, and looking first to the effect of a change of the velocity, and after- 
wards to that of the change of the weight, it becomes evident that these effects 
are to be multiplied together. 

t The quantity of motion communicated to a body does not affect the duration 
of the motion. If but little motion is communicated, the body will move slowly. 
If a great degree is imparted, it will move rapidly. But in both cases the motion 
will continue until it is destroyed by some external force. 

% The resistance of the air, as is stated in No. 41, affects the momentum of all 
bodies. As this resistance is proportioned to the extension of a body, it follows 
that it affects large bodies more powerfully than small ones of equal weight. In all 
nice calculations, allowance is made for the resistance of the medium in which 
bodies are supposed to move. 

Give some instances of perpetual motion in nature. 113. What is the momen- 
tum of a body ? Notes. — How can it be proved that the momentum is ascertain- 
ed by multiplying, not by adding ? Does the quantity of motion communicated to 
a body affect the duration of the motion? If but little motion is communicated, 
how will the body move? If a great degree ? How long will the motion continue ? 
Does the resistance of the air affect the momentum of a body ? To what is this 
resistance proportioned ? What follows from this ? What allowance is made in 
all nice calculations? 114. How can the momentum of a body be ascertained? 
How can a light body be made to have a greater momentum than a heavy one ? 
Give an instance of this. Does the momentum depend alone upon the velocity ? 



30 NATURAL PHILOSOPHY. 

greater must be the force to stop it ; and of course the 
greater the force with which it will strike against an- 
other body. 

115. By the action of bodies is meant the effect 
which they produce upon other bodies. By reaction 
is meant the effect which they receive from the bodies 
on which they act. Thus, when a body in motion 
strikes against another body, it acts upon it, or produ- 
ces action ; but it also meets with resistance from the 
body which is struck, and this resistance is the reac- 
tion of the body. 

116. Action and re-action are always equal, but in 
opposite directions. 

Experiments to show the mutual action and reaction of bodies, 

are made with both elastic and non-elastic bodies. 

s ' ' [See No. 70, and the note connected with it.] Fig. 

/*" 3 represents two ivory balls, A and B, of equal 
weight, &c suspended by threads. If the ball A be 
drawn a little on one side and then let go, it will 
strike against the other ball B, and drive it off to 
• B a distance equal to that through which the first ball 
fell ; but the motion of A will be stopped, because, 
when it strikes B it receives in return a blow equal to that which 
it gave, but in a contrary direction, and its motion is thereby stopped, 
or, rather, given to B. Therefore, when a body strikes against an- 
other, the quantity of motion communicated to the second body is lost 
by the first ; but this loss proceeds, not from the blow given by the 
striking body, bat from the reaction of the body which it struck. 

Fig. 4 represents six ivory balls, of equal weight, suspended by 
threads. If the ball A be drawn out of the perpendicular, and let 
fall against B, it will communicate its motion to B, and receive a 
reaction from it which will stop its own motion. 
Fl s- 4 « But the ball B cannot move without moving C, it 

will therefore communicate the motion which it re- 
ceived from A. to C, and receive from C a reac- 
tion which will stop its motion. In like manner the 
motion and reaction are received by each of the 
I balls, D, E, F ; but as there is no ball beyond F to 
F react upon it, F will fly off. 

IV. B. This experiment can be accurately per- 
formed by those bodies only which are perfectly elastic. 

Fig. 5. Fig. 5 represents two balls of clay, (which are notelas- 

*■ h A J tic) of equal weight, suspended by strings. If the ball 

/ ' \ D be raised and let fall against E, only part of the motion 

J I \ of D will be destroyed by it, (because the bodies are non- 

*9i i • elastic, and the two balls will move on together to d 

* d * *> an( I e i which are less distant from the vertical line than 

] 15. What is meant bv action? Reaction? Illustrate this. 116. How demo- 
tion and reaction compare? Explain Fig. 3d. Fig. 4th. Fig. 5th. 



MECHANICS. 31 

the ball D was before it fell. Still, however, action and reaction 
are equal, for the action on E is only enough to make it move 
through a smaller space, but so much of D's motion is now also de- 
stroyed.* 

117. It is upon the principle of action and reaction, 
that birds are enabled to fly. They strike the air with 
their wings, and the reaction of the air propels them, 
and they are enabled to rise, fall, or remain stationary 
at will, by increasing or diminishing the force of the 
stroke of their wings.f 

It is likewise upon the same principle of action and reaction, that 
fishes swim, or, rather, make their way through the water, name- 
ly, by striking the water with their fins.i 

Boats are also propelled by oars on the same principle, and the oars 
are lifted out of the water, after every stroke, so as completely to pre- 
vent any reaction in a backward direction. 

118. The word reflected means turned bach. Mo- 
tion, therefore, which is turned back is called reflected 
motion. Thus, when a ball is thrown against a hard 
wall, it rebounds, or is turned back. This return of 
the ball is called reflected motion, and it is caused 
by the reaction of the wall against which it struck. 
Reflected motion, therefore, is caused by Reaction. 

119. As reflected motion is caused by reaction, and 
reaction is caused by elasticity, it follows that reflected 
motion is always greater in those bodies which are 
most elastic. For this reason, a ball filled with air, 

* Figs. 3 and 4, as has been explained on the preceding page, show the effect of 
action and reaction in elastic bodies, and Fig. 5 shows the same effect in non-elas- 
tic bodies. When the elasticity of a body is imperfect, an intermediate effect 
will be produced ; that is, the ball which is struck will rise higher than in case of 
non-elastic bodies, and less so than in that of perfectly elastic bodies ; and the 
striking ball will be retarded more than in the former case, but not stopped com- 
pletely, as in the latter. J They will, therefore, both move onwards after the blow, 
but not together, or to the same distance ; but in this, as in the preceding cases, 
the whole quantity of motion destroyed in the striking ball, will be equal to that 
produced in the ball struck. 

f The muscular power of birds is much greater, in proportion to their weight, 
than that of man. If a man were furnished with wings sufficiently large to enable 
him to fly, he would not have sufficient strength, or muscular power, to put them 
in motion. 

% The power possessed by fishes, of sinking or rising in the water, is greatly as- 
sisted by a peculiar apparatus furnished them by nature, called an air-bladder, by 
the expansion or contraction of which, tbey rise or fall, on the principle of spe- 
cific gravity. 

117. Upon what principle do 'birds fly? Explain how. Upon what prin- 
ciple do fishes swim? Upon what principle do boats move upon the water? Ex- 
plain how. 118. What does the word reflected mean? What is reflected mo- 
tion? 119. In what bodies is reflected motion the greatest? 



32 NATURAL PHILOSOPHY. 

rebounds better than one stuffed with bran or wool, 
because its elasticity is greater. For the same reason, 
balls made of Caoutchouc, or India Rubber, will re- 
bound more than those which are made of other sub- 
stances. 

120. The word incident, or incidence, means falling 
upon, or directed towards. Incident motion therefore 
is motion directed towards any particular object. Re- 
flected motion is the same motion turned back. When 
a ball strikes against a wall, it is called the inci- 
dent ball ; and when it rebounds from the wall it is 
called the reflected ball. 

121. The angle* of incidence is the angle formed by 

* As this book may fall into the hands of some who are unacquainted with the 
meaning of angle, perpendicular, the divisions of a circle, &c. a few explanation! 
are here subjoined. 

1. An angle is the opening made by two lines which meet together in a point. 
The size of the angle depends upon the opening, and not upon the length of the 
lines. 

2. A circle is a perfectly round figure, every part 
of the outward edge of which is equally distant 
from a point within, called the centre. [See Fig. 




6 t 



The straight lines drawn from the centre to 



the circumference are called radii. [The singu- 
lar number of this word is radius.] Thus, in Fig. 
6, the lines CD, CO,CR, and CA,are radii. 

4. The lines drawn through the centre, and ter- 
minating in bolh ends at the circumference, are 
called diameters. Thus, in the same Figure, D A 
is a diameter of the circle. 

5. The circumference of all circles is divided in- 
to 360 equal parts, called degrees. The diameter of a circle divides it into two 
equal parts of 180 degrees each. 

6. AH angles are measured by the number of degrees which they contain. Thus 
in Fi". 6, the angle R C A as it includes one quarter of the circle, is an angleof 
90 degrees, which is a quarter of 360. And the angles R C O and O C D are 
angles of 45 degrees. 

7. Angles of 90 degrees are right angles ; angles of less than 90 degrees, acute 
angles, and angles of moie than 90 degrees are culled obtuse angles. Thus, in 
Fig. 6, R C A is a right angle, OCR acute, and OCA obtuse angles. 

8. A perpendicular line is a line which makes an angle of 90 degrees on each 
side of any other line or surface ; therefore, it « ill incline neither to the one side 
nor to the other. Thus, in Fig. 6, R U is perpendicular to I) A. 

Give an instance to illustrate this 120. What does the word incident, or inci- 
dence mean ? What is incident motion ? What is reflected motion ? What is 
the ball called that strikes against a wall ? When it rebounds ? 121. What is the 
ano-le of incidence? (Note — 1. What is an angle? Upon what does the size of 
an°angle depend ? 2. What is a circl" ? 3. What are radii? What lines in Fig. 6 
are radii ? 4. What are diameters? In Fig. 6, what line is the diameter ? 5. 
How is the circumference of all circles divided? Into how many parts doea 
the diameter of a circle divide it ? 6. How are all angles measured ? Illustrate 
this by Fig. 6. 7. How many degrees do right angles contain ? Acute ? Obtuse ? 
Illustrate these angles by Fig. 6. 8. What is a perpendicular line ? What line ia 
perpendicular in Fig. 6? 



MECHANICS. o* 

the line which the incident body makes in its passage 
Fig. 7. towards any object, and a line per- 

pendicular to the surface of the 
object. Thus, in Fig. 7, the line 
ABC represents a wall, and P B 
a line perpendicular to its surface. 
O is a ball moving in the direction 
of the dotted line, O B. The an- 
gle O B P is the angle of incidence. 

122. The angle of reflection, is the angle formed by 
the perpendicular, and the line made by the reflected 
body in its passage, from the surface against which it 
struck. Thus, in Figure 7th, the angle P B R is the 
angle of reflection. 

123. The angles of incidence and reflection are al- 
ways equal to one another. Thus, in Figure 7th, the 
angle of incidence, O B P and the angle of reflection 
P B R are equal to one another; that is, they con- 
tain an equal number of degrees. 

124. From what has now been stated with regard 
to the angles of incidence and reflection, it follows, 
that when a ball is thrown perpendicularly against an 
object, it will return in the same direction ; but if it 
is thrown obliquely, it will return obliquely on the 
opposite side of the perpendicular. The more obliquely 
the ball is thrown, the more obliquely it will rebound.* 

9. The tangent of a circle is a line which touches the circumference, without 
cutting it when lengthened at either end. Thus, in Fig. 6 the line T is a tangent. 

10. A square is a figure having four equal sides, and four equal angles. These 
will always be right angles. [See Fig. 8th. 

11. A parallelogram is a figure whose opposite sides are equal and parallel. 
[See Figs. 9 and 10.] A square is also a parallelogram. 

12. A rectangle is a parallelogram whose angles are right angles. 

13. The diagonal of a square, of a parallelogram, or a rectangle, isa line drawn 
through either of them, and terminating at the opposite angles. Thus, in Figs. 
8, 9 and 10, the line AC is the diagonal of the square, parallelogram, or rectangle. 

* It is from a knowledge of these facts that skill is acquired in many different 
sorts of games, as Billiards, Bagatelle, &c. 

9 What is a tangent ? What line is a tangent in Fig. 6 ? lO.What is a square ? 
11. What is a parallelogram ? 12. A rectangle ? 13. What is a diagonal? What 
lines are diagonal in Figs. 8, 9 and 10 ?) 121. Explain the angle of incidence by the 
figure. 122. What is the angle of reflection ? Illustrate this by Fig. 7. 123. How 
do the angles of incidence and reflection compare with each other? Illustrate this 
by Fig. 7. 124. What follows from what has been stated with regard to the an- 
gles of incidence and reflection.) 
4* 



34 



NATURAL PHILOSOPHY. 



SECTION III. 

Mechanics or the Laws of Motion continued. — Corn-pound 
Motion. 



125. Compound motion is that which is caused by 
the operation of two or more forces at the same time. 

When a body is struck by two equal forces in opposite directions 
it will remain at rest. 

126. A body struck by two forces in different di- 
rections, will move in a line between them. This line 
will be the diagonal of a parallelogram, having for its 
side the lines through which the body would pass, if 
actuated by each of the forces separately. 

Illustration 1st. Fig. 8 represents a ball struck by the two 
equalforces, X and Y. In this figure, the forces are inclined to 
each other at an angle of 90 degrees, or a right 
angle. The force X would send it from C to B, 
and the force Y would send it from C to D. As 
it cannot obey both, it will go between them to A, 
and the line C A, through which it passes, rep- 
resents the diagonal of the square, A B C D. 
The time occupied in its passage from C to A will 
be the same as the force X would require to 
send it to B, and the force Y to send it to D. 

Illustration 2d. If the two forces acting on 
a body are unequal, but still operate at right 
angles to each other, the body will move 
from C to A as represented in Fig. 9 ; in 
which it is to be observed that the force Y is 
as much greater than the force X, as the 
length of the side A B of the rectangle A B 
C D, exceeds the length of the side C B. 

Illustration 3d. When two forces op- 
erate in the direction of an acute angle, 
[See Fig. 10.] the body will move, as rep- 
resented by C A in the parallelogram A 
BCD. 






125. What is compound motion ? In what direction will a body, struck by two 
equal forces in opposite directions, move ? 126. In different directions? What 
is this line called? Illustrate these first, by Fig. 8. which represents a ball struck 
by twoequal forces in different directions. Second, by Fig. 9, which represents 
a ball struck by two unequal forces, acting at right angles. Third, by Fig. 10, 
where the forces operate in the direction of an acute angle. 



MECHANICS. 35 

Illustration 4th. If the forces operate in the direction of an ob- 
tuse angle, the body will move as represented by D B ia the same 
figure. 

127. Circular motion, is motion in a circular di- 
rection, and is caused by two forces operating at the 
same time, by one of which it is projected forward in 
a straight line, while by the other it is confiued to a 
fixed point. 

Illustration. The whirling of a ball, fastened to a string held by 
the hand, is an instance of circular motion. The ball is actuated 
by two forces, namely, the force of projection, and the string which 
conrines it to the hand. The two forces act at right angles to each oth- 
er, and, (according to A*0.1*26.) the ball will move in the diagonal of a 
parallelogram. But. as the force which confines it to the hand only 
keeps it within a certain distance, without drawing it nearer to the 
hand, the motion of the ball will be through the diagonals of an infi- 
nite number of parallelograms, formed by every part of the circle. 

T-2S. The centre of motion is the point to which a 
revolving body is confined. But when the body is not 
of a size or shape to allow every point to revolve in 
the same plane, the line round which it revolves is 
called the axis of motion. The axle of a wheel is the 
axis of the motion of the wheel. The centre or axis 
of motion is not always in the middle of a body. 

JCJ9. The force which confines a body to the centre 
round which it moves, is called the Centripetal* force. 
The force which compels a body to fly off from the 
centre, is called the Centrifugal* force. These are 
called central forces. 

130. If the centrifugal force of a revolving body be 
destroyed, the body will immediately approach the 
centre which attracts it: but if the centripetal force 
be destroyed, the body will fly off in the direction of 
a tangent of the circle which it described in its mo- 
tion. [See Fig. 6,] 

* The word centripetal means seeki g the centre, and centrifugal means firing 
from the centre. The centrifugal force is sometimes called the projectile force. 
In circular motion, these two forces constantly balance each other •, otherwise the 
revolving body will either approach theceatre or recede from it, according as the 
centripetal or centrifugal fure^ is the stronger. 

Fourth, by Fig. 10, where the forces operate in the direction of an obtuse angle. 
127. What is circular motion : How is it caused r Illustrate this. 125. What 
is the centre of motion : What is the axis of motion ? 129. What is the centripe- 
tal force : What is the centrifugal force ? Wnat are the ce irinetal and centri- 
fugal forces called ? 1:10. Wha« would be the consequence if the centrifugal 
and centripetal ffrce3 were destroyed, or did not balance each other: What is 
the meaning of the words centripetal and centrifugal ? 



36 NATURAL PHILOSOPHY. 

Illustration. If a mop filled with water is turned swiftly round 
by the handle, the threads which compose the head will fly off from 
the centre ; but being confined to it at one end, they cannot part 
from it ; whilst the water they contain, being unconfined, is thrown 
off in straight lines. [See No. 137.] 

131. The middle point of a body is its centre of 
magnitude. 

132. The centre of gravity is the point about which 
all the parts balance each other. The centre of grav- 
ity, therefore, is not in the same spot with the centre 
of magnitude, unless the body is of the same density. 

133. The centre of motion is the point round which 
all the parts of a body move. But the centre of mo- 
tion is generally supposed to be at rest. Thus the axis 
of a spinning top is stationary, while every other part 
is in motion around it. The axis of motion and the 
centre of motion are terms which relate only to circu- 
lar motion. 

134. Those parts of a body which are farthest from 
the centre of motion, move with the greatest velocity; 
and the velocity of all the parts diminishes, as their dis- 
tance from the axis of motion diminishes. 

Illustration 1st. Fig. 11 represents the vanes of a windmill. 
The circles denote the paths in which the different parts of the 
Fig. ii. vanes move. M is the centre or axis of 

motion around which all the parts revolve. 
The outer part revolves in the circle D E 
F G, another part revovles in the circle H 
UK, and the inner part in the circle L N 
O P. Consequently, as they all revolve 
around M in the same time. The velocity 
of the parts which revolve in the outer cir- 
cles D E F G and H I J K is as much great- 
er than the velocity of the part which re- 
volves in the inner circle, L N O P, as the 
outer circles are larger than the inner ones. 
As the earth revolves round its axis, it follows,from the preced- 
ing illustration, that the portions of the earth which move most 
rapidly are nearest to the equator, and that the nearer any portion 
of the earth is to the poles, the slower will be its motion. 

131. What is the centre of magnitude ? 132. What is the centre of gravity f 
When are the centres of magnitude and gravity in the same spot? 133. What is 
the centre of motion ? Is the centre of motion supposed to be at rest, or does it 
move ? To what do the terms centre of motion and axis of motion relate? 134. 
What parts of a body move with the greatest volocity ? In what proportion does 
the velocity of all the parts diminish ? What does Fig. 11 represent? What fol- 
lows with regard to the motion of the earth, from the illustration of Fig. 11 ? 




MECHANICS. 37 

135. Motion, either in a circle or ellipsis, or any 
other curve line, must be the result of the action of two 
forces ; for, the impulse of one single force always pro- 
duces motion in a right or straight line. 

136. A ball thrown in a horizontal direction is al- 
ways influenced by three forces ; namely, first, the 
force of projection, (which gives it the horizontal di- 
rection ;) second, the resistance of the air through 
which it passes, which diminishes its velocity, without 
changing its direction ; and third, the force of gravity 
which finally brings it to the ground. 

137. The power of gravity, and the resistance of 
the air, being always greater than any force of pro- 
jection we can give a body, the force of projection is 
gradually overcome, and the body brought to the 
ground. The stronger the projectile force, the farther 
the body will go before it falls. For this reason, a 
shot fired from a cannon will go much farther than a 
stone thrown from the hand.* 

Illustration. Fig. 12 represents a cannon, loaded with a ball, and 
placed on the top of a tower, at such a height as to require just 
Fig. 12 three seconds for another ball to 

descend perpendicularly. Now sup- 
pose the cannon to be fired in a hor- 
izontal direction, and at the same 
instant the other ball to be dropped 
toward the ground. They will 
both reach the horizontal line at the 
base of the tower at the same in- 
stant. In this figure a represents 
the perpendicular line of the falling ball. C b the curvilinear path 

* The action of gravity being always the same, the shape of the curve of every 
projectile (See No 39.) depends on the velocity of its motion. But, whether this 
velocity be great or small, the moving boay, if thrown horizontally from the same 
elevation, will reach the ground at thesHme instant. Thus a ball from a cannon, 
with a charge sufficient to throw it half a mile, will reach the ground at the same 
instant of time that it would, had the charge been sufficient to throw it one, two, 
or six miles, from the same elevation. The distance to which a ball will be pro- 
jected, will depend entirely on the force with which it is thrown, or on the veloci- 
ty of its motion. If it moves slowly, the distance will be short — if more rapidly, 
the space passed over in the same time will be greater ; hut in both cases the de- 
scent of the ball towards the earth, in the same time, will he the same number of 
feet, whether it moves fast or slow, or even whether it move forward at all, or not. 

• 135. Of what is motion in a circle or curve line always the result ? Why ? 136. 
How many forces act upon a ball thrown in a horizontal direction ? W hat are 
they ? 137 Why do bodies fall to the ground ? Why do some bodies ?o farther 
than others before they fall ? What does Fig. 12 represent ? Note. — Upon what 
does the shape of the curve of eVery projectile depend ? Does the time of the 
descent, if thrown horizontally, depend upon the velocity? Illustrate this, 
Upon what does the distance, to which a ball may be projected , depend ? 




38 NATURAL PHILOSOPHY. 

of the projected ball, 3 the horizontal line at the base of the tower. 
During the first second of time, the falling ball reaches 1, the next 
second 2, and at the end of the third second it strikes the ground. 
Meantime, that projected from the cannon, moves forward with 
such velocity, as to reach 4 at the same time that the falling ball 
reaches 1. But the projected ball falls downward exactly as fast as 
the other, since it meets the line 1 4, which is parallel to the hori- 
zon, at the same instant. During the next second the ball from 
the cannon reaches 5, while the other falls to 2, both having de- 
scended through the same space. During the third second the pro- 
jected ball will have spent nearly its whole force, and therefore its 
downward motion will be greater while the motion forward will be 
less than before. 

From hence it appears that the horizontal motion does not in the 
least interfere with the action or the effect of gravity ; but that the 
projectile descends with the same rapidity while moving forward 
that it would if its motion was perpendicular to the horizon. This 
is the necessary result of the action of two forces, according to the 
principle stated in No. 126. 

138. A projectile is any body thrown into the air, as 
a rocket, a ball from a gun, or a stone from the hand. 

139. Projectiles form a curve line in their descent. 
The force of projection being strongest, when they 
are first impelled, is constantly weakened by the re- 
sistance of the air, and the force of gravity, as the body 
proceeds. The direction, therefore, of their motion is 
gradually changed from a horizontal to a perpendicular 
direction. 

Illustration. In Fig. 13 the force of projection would carry a ball 
from A to D, while gravity would bring it to C. If these two forces 
alone prevailed, the ball would pro- 
ceed in the dotted line to B (according 
to the principle stated in number 126.) 
But as the resistance of the air ope- 
rates in direct opposition to the force 
of projection instead of reaching the 
ground at B, it will fall somewhere 
E C about E. 

It is calculated that the resistance of the air to a cannon ball of 
two pounds weight, with the velocity of two thousand feet in a sec- 
ond, is more than equivalent to sixty times the weight of the ball. 

140. When a body is thrown upward obliquely, its 

What follows from this ? 138. What is a projectile ? 139. What line do pro- 
jectiles form in their descent ? Why is the direction of their motion gradually 
changed from a horizontal to a perpendicular direction ? Illustrate this hy Fig. 
13. How g;eat is the resistance of the air calculated to be to a canon ball of two 
pounds weight, with the velocity of 2000 feet in a second ? 140. In what direct 
tion will a body move, when it is thrown upwards obliquely ? 





MECHANICS. 39 

course will be in the direction of a curve-line, called a 
Fig. 14. parabola,* [See Fig. 14] but when it 
is thrown perpendicularly upwards, it 
will descend perpendicularly, because 
the force of projection and that of 
gravity are in the same line of direc- 

a.* tion. 

141. The random of a projectile is the horizontal 
distance from the place whence it is thrown, to the 

* The science of gunnery is founded upon the laws relating to projectiles. The 
force of gunpowder is accurately ascertained, and calculations are predicated up- 
on these principles, which enable the engineer to direct his guns in such a man- 
ner as to cause the fall oftheshotor shells in the very spot where he intends. 
The knowledge of this science saves an immense expenditure of ammunition, 
which would otherwise be idly wasted without producing any effect. In attacks 
upon towns and fortifications, the skilful engineer knows the means he has in his 
power, and can calculate, with great precision, their effects. It is in this way 
that, the art of war has been elevated into a science , and much is made to de- 
pend upon skill, which, previous to the knowledge of these principles, depended 
entirely upon physical power. It is likewise by the same means that wars are 
rcndereit much less sanguinary in modern limes. The force with which balls are 
thrown by gunpowder is measured by an instrument called the Ballistic pendu- 
lum. It consists of a large stock of wood suspended by a rod in the manner of a 
pendulum. Into this block the balls are fired, and to it they communicate their 
own motion. Now the weight of the block, and that of the ball being known, and 
the motion or velocity of the block bein;; determined by machinery, or by observa- 
tion, the elements are obtained by which the velocity of the ball may be found : for, 
the weight of the ball is to the weight of the block as the velocity of the block is to 
the velocity of the ball. By this simple apparatus, many facts relative to the art 
of gunnery may be known. If the ball be fired at different distances, from the 
same gun, it will be seen how much resistance the atmosphere opposes to its force 
atsuch distances. Rifles and guns of smooth bores may be tested, as well as the 
various charges of powder best adapted to different distances and different guns. 
These, and a great variety of other experiments, useful to the practical gunner, 
or sportsman, may be made by this simple means. 

With respect to the velocity of balls impelled by gunpowder, it has been found 
that, with a common charge, from a musket, this is about 1650 feet per second, 
when first discharged. The utmost velocity that can be given to a cannon ball, is 
2000 feet per second ; and this only at the moment of its leaving the gun. 

In order to increase the velocity from 1650 to 2000 feet, one half more powder is 
required : and even then, at a long shot, no advantage is gained ; since, at the dis- 
tance of 500 yards, the greatest velocity that can be obtained is only 1200 or 1300 
feet per second. Great charges of powder are therefore not only useless, but dan- 
gerous ; for, though they give little additional force to the ball, they hazard the 
lives of many by their bursting power. 

Experiment has also shown, that, although long guns give a greater velocity to 
the shot than short ones, still, that on the whole, short ones are preferable ; and, 
accordingly, armed ihips are now almost invariably furnished with short guns, 
called carronades. 

The length of sporting guns has also been greatly reduced, of late years. For- 
merly, the barrels were from four to six feet in length ; but the best fowling pieces 
of the present day have barrels of two feet, or two and a half, only, in length. 
Guns of about this length are now universally employed for such game as wood- 
cocks, partridges, grouse, and such birds as are taken on the wing, with the ex- 
ceptions of ducks and wild geese, which require longer and heavier guns. 

When will a ball descend in the same direction in which it ascended? Why? 
141. What is the random of a projectile ? 



40 



NATURAL PHILOSOPHY. 



place where it strikes. The greatest random takes 

Fig. 15. place at an angle of 45 degrees — that 

is, when a gun is pointed at this angle 

with the horizon, the ball is thrown 

to the greatest distance. Fig. 15 

represents a gun or a carronade, 

from which a ball is thrown at an 

i^. angle of 45 degrees with the hori- 

c zon. 

142. When the centre of gravity of a body (See No. 
132) is not supported, the body will fall. 

143. The base of a body is its lowest side. The 




Fig. 16. 



n 



base of a body standing on wheels or 
legs, is represented by lines drawn 
from the lowest part of one wheel or 
\eg, to the lowest part of the other 
wheel or leg. Thus, in Figures 16 
and 17, D E represents the base of 
of the table. 

line drawn from the centre of 



D E 

the wagon and 

144. Whenever a 
gravity and perpendicular to the horizon falls within 
the base of a body, the body will stand, but when that 
line falls outside of the base, the body will fall or be 
overset. This line is called the line of direction, be- 
cause it is the line which the centre of gravity would 
describe, if the body were suffered to fall. 

Illustration. Fig. 17 represents a loaded wagon on the declivity 
of a hill. The line C F represents the horizon. D E the base of 
the wagon. If the wagon be loaded in such a manner that the cen- 
tre of gravity is at B, the perpendicular B D 
falls within the base, and the wagon will stand. 
But if the load be altered so that the centre of 
gravity is raised to A, the perpendicular AC will 
fall o.Uside of the base, and the wagon will be 
overset. From this it follows that a wagon, or 
any carriage, will be most firmly supported when 
the centre of gravity falls exactly between the 
C F wheels ; and that is the case on a level road. The 

centre of gravity, in the human body, is between the hips, and 
the base is the feet and the space between them. 



Fis. 17. 




At what angle does the greatest random take plnce ? 142. When will a body 
fall ? What is the base of a body ? In Fig. 16 and 17, what represents the base ? 
144. When will a body stand ? When will it fall ? Illustrate this by Fig. 17. 
What follows from this? Where is the centre of gravity in the human body ? 
Where is the base ? 



MECHANICS. 41 

So long as we stand uprightly, the line of direction falls within 
this base. When we lean on one side, the centre of gravity, not 
being supported, we no longer stand firmly. 

A rope-dancer performs all his feats of agility by dexterously 
supporting the centre of gravity. For this purpose he carries a 
heavy pole in his hands, which he shifts from side to side as he al- 
ters his position, in order to throw the weight to the side which is 
deficient; and thus, by changing the situation of the centre of grav- 
ity, he keeps the line of direction within the base, and he will not 
fall.* 

145. A spherical body will roll down a slope, because 
the centre of gravity is not supported.! 

146. When a body is of uniform density, the centre 
of gravity is in the same point; when one part of the 
body is composed of heavier materials than another 
part, the centre of gravity, (being the centre of the 
weight of the body,) no longer corresponds with the 
centre of magnitude. Thus the centre of gravity of 
a cylinder plugged with lead, is not in the same spot as 
the centre of magnitude. Bodies, therefore, consist- 
ing of but one kind of substance, as wood, stone, or 
lead, and whose densities are consequently uniform, 
must stand more firmly, and be more difficult to over- 
set, than bodies composed of a variety of substances, 
of different densities, which may throw the centre of 
gravity on one side. 

* The shepherds in the south of France afford an interesting instance of the 
application of the art of balancing to the common business of life. These men 
walk on siilts from three to four feet high, and their children, when quite young, 
are taught to praciise the same art. By means of these odd additions to the length 
of the leg, their feet are kept out of the water, or the heated sand, and they are, 
also, enabled to see their sheep at a greater distance. They use these stilts with 
groat skill and care, and run, jump, and even dance on them with great ease. 

t A cylinder can be made to roll up a slope, by plugging one side of it with 
lead ; the body being no longer of a uniform density, the centre of gravity is re- 
moved from the middle of the body to some point in the lead, as that substance is 
much heavier than wood. Now, in order that the cylinder may roll down the 
plane, as it is here situated, the centre of gravity must rise, which is impossible ; 
the centre of gravity must always descend in moving, and will descend by the near- 
est and readiest means, which will be by forcing the cylinder up the slope, until 
the centre of gravity is supported, and then it stops. 



How is it that rope-dancers perform their feats of agility ? 145. Wliy do spher 
ical bodies roll down slopes ? How can a cylinder be made to roll up a slope? 
How does this affect it? 146. Where is the centre of gravity in a body of uni- 
form density ? Do the centre of gravity and the centre of magnitude corres- 
pond when one part of a body is composed of heavier materials than another? 
What bodies must stand more firmly than others ? Why? 

5 



42 NATURAL PHILOSOPHY. 

147. Bodies that have a narrow base are easily 
overset; for if they are the least inclined, the line of 
direction will fall outside of the base, and their centre 
will not be supported.* 

148. The broader the base, and the nearer the cen- 
tre of gravity to the ground, the stronger will be the 
edifice. 

For this reason a pyramid, as it has a broad base and but little 
elevation, is the firmest and most durable of all structures. 

149. When two bodies are fastened together, they 
are to be considered as forming but one body, and have 
but one centre of gravity. If the two bodies are of 
equal weight, the centre of gravity is in the middle of 
the line which unites them. But if one be heavier than 
the other, the centre of gravity is as much nearer to 
the centre of the heavy one than to the light one, as the 
heavy exceeds the light one in weight. 

Illustration. Fig. 18 represents a rod or pole with an equal weight 
Fig. 18. fastened at each end : the centre of gravity is 

at A, the middle of the rod, and whatever 
A ffB yt supports this centre will support both the bod- 
ies and the pole. 

Fig. 19. Fig. 19 represents a rod or pole with an unequal 

weight at each end. The centre of gravity is at 
C nearer to the larger body. 

Fig. 20 represents a Fig. 20. 

rod or pole with unequal 
weights at each end, but ^ 
the larger weight exceeds the less in such a 
degree that the centre of gravity is within 
the larger body at C. 



* A person can carry two pails of water more easily than one, because they 
balance each other and the centre of gravity remains supported by the feet. But 
a single pail throws the centre of gravity on one side, and renders it more diffi- 
cult to support the body. 





147. Why do bodies which have a narrow base overturn more easily than 
those which have broad bases? Why can a person carry two pails of water 
more easily than one? 148. Why is a pyramid the firmest and most durable of 
all structures ? 149. If two bodies of equal weight are fastened together, where 
is the centre of gravity ? If one be heavier than the other ? What doe3 figure 
ISreprosent? Fig. 19? Fig. 20? 



MECHANICS. 43 

SECTION VI. 

Resultant Motion. 

150. Resultant motion is the effect, or result of two 
motions resolved into one. 

Illustration. If two men are sailing in two boats, in the same di- 
rection, and at the same rate, and one toss an apple to the other, the 
apple would appear to pass directly across from one to the other, in 
a line of direction perpendicular to the side of each boat. But its 
real course is through the air in the diagonal of a parallelogram, 
formed by the lines representing the course of each boat, and per- 
pendiculars drawn to those lines from the spot where each man 
stands as the one tosses and the other catches the apple. In Fig. 21 
Fi(T 2i. tne l mes A B and C D represent the course of 

£ p each boat ; E is the spot where the man stands 

A - b wno tosses the apple : while the apple is in its 

passage, the boatshave passed from E and G to H 
D and F respectively. But the apple having a mo- 
tion with the man that would carry it from E toH 
and likewise a projectile force which would car- 
ry it from E to G, cannot obey them both, but will pass through the 
dotted line E F, which is the diagonal of the parallelogram E G F 
H, according to the principle in No. 126. 

On the principle of resultant motion, if two ships in an engage- 
ment are sailing before the wind, at equal rates, the aim of the gun- 
ners will be exactly as though they both stood still. But if the gun- 
ner fires from a ship standing still, at another under sail, or a 
sportsman fires at a bird on the wing, each should take his aim a 
little forward of the mark, because the ship and the bird will pass a 
little forward while the shot is passing to them. 



SECTION VII. 

The Pendulum. 
151. The Pendulum* consists of a weight, or ball of 

* The pendulum was invented by Galileo, a great astronomer of Florence, in 
the beginning of the seventeenth century. Perceiving that the chandeliers sus- 

150. Of what is resultant motion the effect? What illustration is given ? Ex- 
plain by fig. 21. 151. Of what does a Pendulum consist ? 




44 NATURAL PHILOSOPHY. 

metal, suspended by a rod, and made to swing back- 
wards and forwards. 

152. When a pendulum swings, it is said to vibrate, 
and its movements are called vibrations. The part of 
a circle through which it moves, is called its arc. The 
attraction of gravity causes its vibrations. 

153. The vibrations of pendulums, of equal length, 
are very nearly equal, whether they move through a 
greater or less part of their arcs. 

Illustration. In figure 22 A B represents a pendulum. D E F 
Fig- 22. fA C the arc in which it vibrates. If the pen- 

dulum be raised to E it will return to F, if 
it be raised to C it will return to D in near- 
ly the same length of time, because that in 
proportion as the arc is more extended, the 
steeper are its beginnings and endings, and, 
therefore, the more rapidly will it fall. 

154. The time occupied in the vibration of a pendu- 
lum, depends upon its length. The longer the pendu- 
lum, the slower are its vibrations. 

155. The length of a pendulum which vibrates sixty 
times in a minute (or in other words which vibrates 
seconds) is about 39 inches. But in different parts of 
the earth this length must be varied. A pendulum to 
vibrate seconds at the equator must be shorter than one 
which vibrates seconds at the poles. 

pended from the ceiling of a lofty church vibrated long and with great uniformi- 
ty, as they were moved by the wind or by any accidental disturbance, he was led 
to inquire into the cause of their motion, and this inquiry led to the invention of 
the pendulum. From a like apparently inconsiderable circumstance arose the 
great discovery of the principle of gravitation. During the prevalence of the 
plague, in the year 1665, Sir Isaac Newton retired into the country to avoid the 
contagion, fitting in his orchard, one day, he observed an apple fall from a tree. 
His inquisitive mind was immediately led to consider the cause which brought the 
apple to the ground, and the result of his inquiry was the discovery of that grand 
principle of gravitation {See Nos.ll, 72) which may be considered as the first and 
most important law of material nature. Thus, out of what had been before the 
eyes of men, in one shape or another, from the creation of the world, did these 
philosophers bring the most important results. 

152. When is a pendulum saidto vibrate? What are its movements called? 
What is meant by its arc ? What causes its vibrations ? 153. How do the vibra- 
tions of pendulums of equal length compare? Illustrate by fig. twenty-two. By 
whom was the pendulum invented » What led him to the discovery? By whom 
was the principle of gravitation discovered? What led him to the discovery? 
154. Upon what does the time of the vibrations of a pendulum depend ? 155. 
What is the length of a pendulum which vibrates sixty time9 in a minute? Do 
different situations affect the vibrations? How can a pendulum which vibrates 
seconds at the equator be made to vibrate seconds at the poles .' 



MECHANICS. 45 

156. A clock is regulated by lengthening or short- 
ening the pendulum. By lengthening the pendulum 
the clock is made to go slower, by shortening it, it will 
go faster. The pendulum of a clock is made longer 
or shorter, by means of a screw beneath the weight or 
ball of the pendulum. The clock itself is nothing more 
than a pendulum connected with wheel-work, so as to 
record the number of vibrations. A weight is attach- 
ed in order to counteract the retarding effects of fric- 
tion, and the resistance of the air. The wheels show 
how many swings or beats of the pendulum, have taken 
place in a given time, because at every beat, the tooth 
of a wheel is allowed to pass. Now if this wheel has 
sixty teeth, it will turn round once in sixty vibrations 
of the pendulum, or in sixty seconds, and a hand 
fixed on the axis of the wheel projecting through the 
dial plate will be the second hand of the clock. Other 
wheels are so connected with the first, and the number 
of teeth in them so proportioned, that the second wheel 
turns sixty times slower than the first, and to this is 
attached the minute hand ; and the third wheel moving 
twelve times slower than the second, carries the hour 
hand. On account of the expansion of the pendulum 
by heat, and its contraction "by cold, clocks go slower 
in summer than in winter, because the pendulum is 
thereby lengthened at that season. 

A watch differs from a clock, in having a vibrating 
wheel instead of a pendulum. This wheel is moved by 
a spring called the hair spring. The place of the weight 
is supplied by another larger spring called the main 
spring. 



SECTION VIII. 

The Mechanical Powers. 
157. The mechanical powers are certain contrivances 

156. How is a clock regulated? What eflfcct has the lengthening of it \ The 
shortening ! What is a clock? Of what use is the weight? What do the 
wheels show .' Why do clocks go slower in summer than in winter? How does 
& watch differ from a clock; 157. What are the mechanical powers f 

5* 



46 NATURAL PHILOSOPHY. 

designed to increase, to diminish, or to alter the di- 
rection of any force. 

158. There are five things which are to be consid- 
ered, in order to understand the power of a machine,* 
Namely : First, The power that acts — Secondly, The 
resistance which is to be overcome by the powers — 
Thirdly, The centre of motion, or as it is sometimes 
called, The fulcrum, (which means a prop or support) 
Fourthly, The respective velocities of the power and 
the resistance ; and Fifthly, The instruments em- 
ployed in the construction of the machine. 

Illustration. The power that acts is the muscular strength of 
men, or animals, the weight and momentum of solid bodies, the 
elastic force of steam, springs, the pressure of the air, &c. 

The resistance to be overcome is the attraction of gravity, or of 
cohesion, the inertness of matter &c. 

The centre of motion or the fulcrum, is the point about which 
all the part- of the body move. 

The velocity, as has already been explained, is represented by the 
time occupied in producing a certain effect. 

The instruments are the mechanical powers which enter into the 
construction of the machine. 

159. There are six mechanical powers, namely, the 
Lever, the Pulley, the Wheel and Axle, the Inclined 
Plane, the Wedge, and the Screw. 

160. The Levert is an inflexible! bar, movable on a 
fulcrum or prop. 

161. There are three kinds of levers, called the first, 



* In order to produce any effect by mechanical means, it is necessary that the 
power employed be greater than the resistance which is to be overcome. Thus, 
for instance, if the weight of a loaded wagon is greater than the strength of the 
horses employed to draw it, they will not be able to move it. 

fThe Lever is made in a great variety of forms, and of many different mate- 
rials- 

:J.By an inflexible bar is meant one which will not bend. The fuWum or prop 
it, likew Ue, constructed in a variety of ways. Sometimes it is merely a stone on 
which a lever in the form of a crow-bar rests. Sometimes it Js a pin passing 
through the lever, &c. 



158. How many things are to be considered in order to understand the power of 
a machine? What is the first; Second? Third? Fourth; Fifth? What is the 
power that acts > What is the resistance to be overcome ? What is the ful- 
crum > What is the velocity ! Wh.it is necessary in order to produce any effect 
by mechanical powers ; What illustration of this is given ! 159. How many me- 
chanical. powers are there; What are they ! 169. What is a lever.' 161. How 
many kinds of levers are there .' How do they differ i 



MECHANICS. 



47 



second, and third kinds, according to the respective 
position of the fulcrum, the power and the weight. 

1(32. In a lever of the first kind, the weight is at one 
end, the power at the other, and the fulcrum between 
them. 

Fig. 23 represents a simple lever of the first kind, 



Illustration. 
Fig 



•23. 




Tv- 



resting on the fulcrum F . and moveable 
upon it. TV is the weight (or heavy 
stone) to be moved, and P is the pow- 
er, (or hand) which moves it. The 
advantage gained in the use of this 
kind of lever is in proportion as the 
distance of the power from the ful- 
crum exceeds that of the weight from 
the fulcrum. Thus, in this figure, if 
the distance between P and F be double that between W and F, 
then a man by the exertion of a force of 100 pounds with the lever 
can move a weight of 200 lbs. From this it follows that the near- 
er the power is applied to the end of the lever most remote from the 
fulcrum the greater is the advantage gained. Thus, in the same 
figure, a greater weight can be moved by the same power when ap- 
plied at B than when it is exerted at P.* 

A balance or pair of scales is a lever of the first kind, with equal 
arms. Steelyards, scissors, pincers, snuffers, and a poker used for 
stirring the fire, are all levers of the first kind. The longer the 
handles of scissors, pincers, &c, and the shorter the points, the more 
easily are they used. 

*It is a fundamental principal in mechanics that what is gained in power is lost 
in time. [See JVu.97.] To illustrate this principle, (Fig. 24.) W represents the 



Pis. 24. 




weight, F the fulcrum, P the power and the bar, 
W F P the lever. To raise the weight W to 0, 
the power P must descend to p. But as the radius 
of the circle in which the power P moves is double 
that of the radius of the circle in which the weight 
W moves, the arc P p is double the arc XV w ; or, 
in other words, the distance P p is double the dis- 
tance of \V w. Now, as these distances are traversed 
in the same time by the power and the weight, re- 
spectively, it follows that the velocity of the pow- 
er -nu^t be double the velocity of the weight ; that 
is, the power mu3t move at the rate of two leet in 
a second, in order to move the weight one foot 
in thosame time. 

This principle applies not only to the lever, but 

to aM the mechanical powers, and to all machines constructed on mechanical 

punciples 

When two weights are equal, and the fulcrum is placed exactly in the centre of 

16-2. What is a levnr of the first kind ! What figure illustrates this ! Explain 
it by the figure ! To what is the advantage, gained by this lever, proportional; 
What follows from this ; What is meant by an inflexible bar; What is a fun- 
damental principle in mechanics ? Illustrate this by the figure? Does this prin- 
ciple npply to all the mechanical powers; When two weights are equal where 
is the fulcrum ! How must the fulcrum and power be placed to make the lever 
act as a mechanics 1 power; Upon what does the force of the lever depend ! Give 
some examples of levers of the first kind. 



48 NATURAL PHILOSOPHY. 

Fi S- 95 « A hammer-lever differs 

P D„ c « only in form from levers of 

Jtfj ^ .^. t *y i,^, m m«n LiiiijuigMi the first kind. A com- 

33 ijl pound lever consists of sev- 

■ eral levers so arranged 

that the shorter arm of one 

may act on the longer arm of the other. (See Jig. 25.) 

163. In a lever of the second kind the fulcrum is 
at one end, the power at the other, and the weight be- 
tween them. 

Illustration. Fig. 26 represents a lever of the second kind. Fis 
Fig. 26. the fulcrum, P the power, and W the weight. 

€The advantage gained by a lever of this kind 
is in proportion as the distance of the power 
„ L , from the fulcrum exceeds that of the weight 

* from the fulcrum. Thus as this figure of the 

£ distance from P to F is four times the distance 

from W to F, then a power of one pound at 

P will balance a weight of four pounds at W. 

This kind of lever explains the manner in which two persons, 

carrying a heavy burthen, (as, for instance, a cask upon a pole,) 

may be made to bear unequal portions of it, by placing it nearer to 

the one than the other. 

Two horses, also, may be made to draw unequal portions of a 
load by dividing the beam attached to the carriage in such a man- 
ner that the weaker horse may draw upon the longer end of the 
beam. 

Oars, rudders of ships, doors turning on hinges, and cutting- 
knives, which are fixed at one end, are constructed upon the princi- 
ple of levers of the second kind.* 

164. In a lever of the third kind, the fulcrum is at 
one end, the weight at the other, and the power is ap- 
plied between them. 

the lever between them, they will mutually balance each other ; or, in other words, 
the centre of gravity being supported, neither of the weights will sink. 

To make the lever act as a mechanical power, I h< • fulcrum must be placed 
near the weight to be moved, snd the power or hand at the greater distance from 
it. The force of the lever, therefore, depends on its length, together with the pow- 
er applied, and the distance of the weight from the fulcrum. 

* It is on the same principle that in raising a window the hand should be ap- 
plied to the middle of the sash, it will then be easily rai ed ; whereas, if the 
hand be applied nearer to one side than the other, the centre of gravity being un- 
supported, will cause the farther side to bear against the frame, and obstruct its 
free motion. 



163. What is a lever of the second kind? What figure illustrates this? To 
what is the advantage gained, by this lever, proportional > Give some examples 
of levers of the second kind. 164. What is a lever of the third kind ? 




MECHANICS. 49 

In levers of this kind the power must always exceed the weight, 
in the same proportion as the distance of the weight from the ful- 
crum exceeds that of the power from the fulcrum. 

Fig. 21, F is the fulcrum, W the weight, and P the power between 
Fig. 27. the fulcrum and the weight; and the power must 

exceed the weight in the same proportion that the 
distance between W and F exceeds the distance 
between P and F. 

A ladder which is to be raised by the strength 

of a man's arms, represents a lever of this kind, 

where the fulcrum is that end which is fixed 

w against the wall, or upon which a man stands, 

the weight may be considered as at the top part of the ladder, and 

the power is the strength applied to the rearing of it. 

The bones of a man's arm, and most of the movable bones of 
animals, are levers of the third kind. But the loss of power in the 
limbs of animals is compensated by the beauty and compactness of 
the limbs. The wheels, in clock and watchwork, and in various 
kinds of machinery, may be considered as levers of this kind, when 
the power that moves them acts on the pinion, near the centre of 
motion, and the resistance to be overcome acts on the teeth at the 
circumference. But here the advantage gained is the change of 
slow into rapid motion. The sails of vessels are constructed on 
the principal of the lever. 

165. The pulley is a small wheel turning on an axis, 
with a string or rope in a groove running around it. 

166. There are two kinds of pulleys, the fixed and 
the movable. The fixed pulley is a pulley fastened to 
the wall or to a beam, and is used only for changing 
the direction of motion. 

Illustration. Fig. 23 represents a fixed pulley. P is a small 
wheel turning on its axis, with a string running round it in a groove. 
Fig. 28. W is a weight to be raised. F is the force or power appli- 
ed. It is evident that by pulling the string at F the weight 
must rise just as much as the string is drawn down. As, 
therefore, the velocity of the weight and the power is pre- 
cisely the same, it is manifest that they balance each other, 
and that no mechanical advantage is gained. [See No. 97.] 
But the pulley is very useful for changing the direction of 
motion. If, for instance, we wish to raise a weight to the 
top of a high building, it can be done with the assistance 
of a fixed pulley, by a man standing below.* A curtain or a 

* The fixed pulley operates on the same principle as a lever of the first kind 
with cqunl arm?, where the fulcrum being in the centre of gravity, the power and 
the weight are equally distant from it, and no advantage is gained. 

In what proportion must the power exceed the weight in this lever I Explain 
Fig. tweiity-sixth. Give some examples of levers of the third kind. 165. 
What is a pulley j 166. How many kinds of pulleys are there > What are they I 
What is a fixed pulley .' Explain Fig. twenty seventh. What advantage is gained 
by this pulley > What is the use of this pulley. Upon what principle does the 
fiied pulley operate ,' 



50 



NATURAL PHILOSOPHY. 




sail, also, can be raised by means of the fixed pulley, without as- 
cending with it, by drawing down the string connected with it. 

167. The movable pulley differs from the fixed pul- 
ley by being attached to the weight; it therefore rises 
ami falls with the weight. 

Illustration. Fig. 29 represents a moveable pulley, with the weight 
Fig. 29. W attached to it by a hook below. One end of the rope 
is fastened at F; and as the power P draws the. weight 
upwards the pulley rises with the weight. Now, in or- 
der to raise the weight one inch, it is evident that both 
sides of the string must be shortened, in order to do 
which the power P must pass over two inches. As the 
velocity of the power is double that of the weight, it 
follows (by number 97) that a power of one pound will 
balance a weight on the moveable pulley of two pounds. 
From which it appears that — 

168. The power gained by the use of pulleys is as- 
certained i y multiplying the number of movable pul- 
leys by 2. 

Illustration. A weight of 72 pounds may be balanced by a pow- 
er of 9 pounds with lour pulleys; by a power of 18 pounds with 
two pulleys; or by a power of 36 pounds with one pulley. But in 
each case the space passed over by the power must be double the 
space passed over by the weight, multiplied by the number of mova- 
ble pulleys. That is, 10 raise the weight one foot, with one pulley, 
the power must pass over (double of one foot, the space passed over 
by the weight multiplied by one, which is equal to) two feet, with 
two pulleys four feet, with four pulleys eight feet. 
Fig. 30 represents a system of fixed and movable pulleys. In 
the block F, there are four fixed pulleys, and in the 
block M there are four movable pulleys, all turn- 
ing on their axes, and rising and falling with the 
weight W. The movable pulleys are connected 
with the fixed ones by a string attached to the hook 
H, passing over the alternate grooves of the pul- 
leys in each block, forming eight cords, and termi- 
nating at the power P. Now to raise the weight 
one foot, it is evident that each of the eight cords 
must be shortened one foot, and, consequently, that 
the power P must descend eight times that dis- 
tance. The power, therefore, must pass over eight 
times the distance that the weight moves. 




167. How docs the movable pulley JiffW from the fixed pulley ? Explain Fig. 
twenty-ninth. 168. How can the power, gained by the use of the movable 
pulleys, be ascertained > What illustration ol this is given .' Whut does Fig. 
thirty represent I 



MECHANICS. 51 

169. Pulleys act on the same principle with the lev- 
er, the deficiency of the strength of the power being 
compensated by its superior velocity. [ See No. 97.] 
Now, as we cannot increase our natural strength, but 
can increase the velocity of motion, it is evideut that 
we are enabled, by pulleys and other mechanical pow- 
ers, to reduce the resistance or weight of any body to 
the level of our strength. 

Practical use of Pulleys. Pulleys are used to raise goods into 
warehouses, and in ships, &c. to draw up the sails. Both kinds of 
pulleys are in these cases advantageously applied ; for the sails are 
raised up to the masts by the sailors on deck by means of the fixed 
pulleys, while the labor is facilitated by the mechanical power of 
the movable nes. 

Both fixed and movable pulleys are constructed in a great varie- 
ty of forms, but the principle on which all kinds are constructed, 
is the same. What is generally called a tackle and fall, or a block 
and tackle, is nothing more than a pulley. Pulleys have, likewise, 
lately been attached to the harness of a horse to enable the driver 
to govern the animal with less exertion of strength. 

It may be observed, in relation to the mechanical powers in gen- 
eral, that power is always gained at the expense of time and veloci- 
ty ; that is, the same power which will raise one pound in one min- 
ute, will raise two pounds in tw T o minutes, six pounds in six minutes, 
sixty pounds in sixty minutes, &c; and that the same quantity of 
force used to raise two pounds one foot, will raise one pound two 
feet, &c. And, further, it may be stated that the product of the 
weight, multiplied by the velocity of the power, will always be equal 
to the product of the power multiplied by the velocity of the weight. 
Hence we have the following rule. The power is in the same pro- 
portion to the weight as the velocity of the weight is to the velocity 
of the power. 

170. The wheel and axle consists of two wheels, one 
of which is smaller than the other, revolving together 
around the same centre of motion. The place of the 
smaller wheel is generally supplied by a cylinder, 
which is called the axle. A cylinder is a round body 
with flat ends. 



169. Fpon what principle do pulleys act ? What advantage U <*ai<ieci by the 
use of pulleys and other mechanical powers.' What are s >me of t,:e practical 
uses of th" pulley >. What is a tackle and fall? Is there any time or velocity 
gained with the power in the mechanical powers ? To whar is the product of the 
weight, multiplied by the velocity of the powei, always equal! What rule is 
given! 170. Of what does the wheel and axle consist? What is a cylinder? 



52 



NATURAL PHILOSOPHY. 



Fig. 31. 




Fi-. 32. 



llhistration . T he 

wheel and axis, though 
made in many forms, 
will easily be under- 
stood by inspecting fig- 
ures 31 and 3*2. In fig. 
31 P represents the larg- 
er wheel where the pow- 
er is applied; C the 
smaller wheel or cylin- 
der, which is generally 
called the axis, and W 
the weight to be raised. 
The advantage gained 
is in proportion as the 
circumference of the 
wheel is greater than 
that of the axis. That 
is, if the circumference 
of the wheel is six times 
the circumference of the axis, then a power of one pound applied at 

the wheel will balance a 
power of six pounds on 
the axis. 

Sometimes the axis is 
constructed with a winch 
or handle instead of the 
wheel, as in fig. 32, or 
with projecting spokes, 
as in fig. 31. 

The principle upon 
which the wheel and ax- 
le is constructed is the 
same with that of the 
other mechanical pow- 
ers, the want of pow- 
er being compensated by velocity. It is evident (from the figures 
31 and 32) that the velocity of the circumference of the wheel is 
as much greater than that of the axle as it is further from the cen- 
tre of motion; for the wheel describes a great circle in the same 
time that the axle describes a small one; therefore the power is in- 
creased in the same proportion as the circumference of the wheel is 
greater than the axle. If the velocity of the wheel is twelve times 
greater than that of the axle, a power of one pound on the wheel 
will support a weight of twelve pounds on the axle. 

171. In connexion with the wheel and axle, it is 
proper to mention the subject of complex wheel-work. 
It has already been stated that the velocity of the wheel 

What figures illustrate the wheel and axle.' Explain. To what is the advantage 
gained in proportion? What does Fig 32 represent ? Fig. 31 ? Upon what princi- 
ple is the wheel and axle constructed ; Explain by figures 31 and 32. 





MECHANICS. 53 

is greater than that of the axis ; and this velocity is in 
proportion to the relative size of the wheel compared 
with that of the axis. Advantage is taken of this cir- 
cumstance in driving machinery where the speed is to 
be increased or diminished. For it is evident that 
when quick motion is to be produced, that if the power 
is applied to the axis, and machinery is attached to the 
wheel, that rapid motion will be produced ; and that, on 
the contrary, if the power be applied to the wheel and 
the machinery to the axis, that slow motion will be pro- 
duced. 

Fig. 33 represents four wheels with their axles, 
each wheel acting on the axle of the adjoin- 
a ing wheel. F is the power applied to the ax- 
le of the wheel d. Now, supposing the cir- 
cumference of each wheel to be six times the 
circumference of each axle, it is evident that 
each time the wheel d revolves it must cause 
the wheel c to make six revolutions, because 
the circumference of the wheel d is six times 
the circumference of the axle of c. In like 
manner the circumferences of the wheels c and b, acting respective- 
ly on the circumferences of the axles of the adjoining wheel, will 
communicate a velocity six times greater than their own, and while 
the wheel d makes one revolution the wheel c will make six, b thir- 
ty-six, and a two hundred and sixteen revolutions. 

Reversing the figure, and applying the power at S which com- 
municates with the circumference of the wheel a, it follows that a 
must perform six revolutions while b is performing one, thirty-six 
while c, and two hundred and sixteen while d performs one revolu- 
tion. It will thus be perceived that a rapid or a slow motion may 
be communicated by various combinations of the wdieels and axle. 

172. The usual way of transmitting the action of the 
axles to the adjoining wheels is by means of teeth or 
cogs, raised on their surfaces. The cogs on the surface 
of the wheels are generally called teeth, and those on 
the surface of the axle are called leaves. The axle 
itself, when furnished with leaves, is called opinion. 



171. How does the velocity of the wheel compare with that of the axle? To 
what is this velocity in propor'.ion ? Is any advantage taken of this, in driving 
machinery where the speed is to be increased or diminished? How would rapid 
motion be produced > Slow motion ? Explain figure 33. 172. What is the usual 
way of transmitting the action of the axles to the adjoining wheels? What are 
the cogs on the surface.of the wheel called > Those on the axle > What is a pinion I 



54 



NATURAL PHILOSOPHY. 




Illustration. Fig. 34 
represents a connexion 
of cogged wheels. The 
wheel B being moved 
by «? string around its 
circamference is a sim- 
ple wheel without teeth. 
Its axis being furnished 
with cogs or leaves to 
which the teeth of the 
wheel D are fitted, com- 
municates its motion to 
D, which, in like man- 
ner, moves the wheel C. 
. . TXT The power P and the 

weight W must be attached to the circumference of the wheel or of 
the axis, according as a slow or a rapid motion is desired. 

173. Wheels are sometimes turned by bands, as in 
Fig. 35. figure 35 ; and the motion communica- 

ted may be direct or reversed by attach- 



jjp i n g the band, as represented in figs. 35 
and 36. When the wheel and the axle 
from which it receives motion are intended to revolve 
in the same direction, the strap is not crossed, but is 
rig. 36. applied as in fig. 35. But when the 

wheel is to revolve in a direction con 
trary to the revolution of the axle, the 
strap is crossed, as in fig. 36. 
174. Different directions maybe given to the motion 

37. Fi?. 38. 




produced by wheels, by varying the position of their ax- 
les, and causing them to revolve in different planes, as 

Expluin Fig. 34. 173. By what, are wheels sometimes turned? What figure 
represents one >. In what way can the motion be madu direct or reversed .' What 
does Fig. 35 represent ? Fig. 36! 174. In what way can different directions be 
given to the motion produced by wheels ? What does Fig. 37 represent '. Fig. 38! 



MECHANICS. 55 

in fig. 37 ; or by altering the shape and position of the 
teeth or cogs, as in fig. 38. 

175. It remains to be observed that the wheel and 
axle are constructed on the same principle with 4he 
lever. The axle acts the part of the shorter arm of 
the lever, the wheel that of the longer arm. 

176. The capstan, on board of ships and other ves- 
sels, is constructed on the principle of the wheel and 
axle. It consists of an axle placed uprightly, with a 
head or drum, pierced with holes for the lever, or lev- 
ers, which supply the place of the wheel. 

Windmills, Lathes, the common windlass, used for drawing wa- 
ter from wells, and the large wheels in mills are all constructed on 
the principle of the wheel and axle. 

177. Wheels are a very essential part to most ma- 
chines ; they are applied in different ways, but when 
affixed to the axle their mechanical power is always in 
the same proportion ; that is, as the circumference of 
the wheel exceeds that of the axle, so much will the 
power be increased. Therefore the larger the wheel 
and the smaller the axis, the greater will be the power 
obtained. 

17S. Fly wheels are heavy wheels used to accumu- 
late power and distribute it equally among all the parts 
of a machine. They are caused to revolve by a force 
applied to the axle; and when once set in motion con- 
tinue by their inertia to move for a long time. As their 
motion is steady and without sudden jerks, they serve 
to steady the power, and cause a machine to work with 
regularity. 

179. Cranks are sometimes connected with the axle 
of a wheel, either to give or to receive its motion. They 
are made by bending the axle in such a manner as to 
form four right angles facing in different directions, as 



175. Upon what principle are the wheel and axle constructed; Explain how. 
176. Upon what principle is the capstan onboard of vessels constructed.' Of 
what does it consist ! What other things are mentioned as constructed upon this 
principle? 177. Are wheels an essential part to most machines! Are they appli- 
ed in more than one way! When they are affixed to the axle, in what proportion 
is the power increased! " 3. What are Fly wheels, and for what are they used? 
How are they made to revolve? When once set in motion, what causes them to 
jnove on for some time ? Of what service are they in a machine ! 179. For what 
are eranks, sometimes, connected with the axle of a wheel >. How are they made i 



56 NATURAL PHILOSOPHY. 

Fig. 39. is represented in fig. 39. This is seen 

in lathes and many other kinds of ma- 
chinery. Cranks are often used to change 
the motion from rectilinear to circular, 
or from circular to rectilinear. 

180. The inclined plane consists of a plain surface 
inclined to the horizon. 

Illustration. Fig. 40 represents an inclined plane. C A its height, 
C B its length, and W a weight which is to be moved on it. The 
Fig. 40. advantage gained by the use of the inclin- 

ed plain is in proportion as the length of 
^ §||t--f'C trie plane exceeds its perpendicular height. 
Thus, in this figure, if the length C B is 

£ ^^ !A four times the height C A, then a power 

of one pound will balance a weight of four 
pounds on the inclined plane. 

181. The greater the inclination of the plane, the 
greater must be its perpendicular height, compared 
with its length, and, of course, the greater must be the 
power to elevate a weight along its surface. 

Instances of the application of the inclined plane are very com- 
mon. Sloping planks or pieces of timber leading into a cellar, and 
on which casks are rolled up and down ; a plank or board with one 
end elevated on a step, for the convenience of trundling wheel-bar- 
rows, or rolling barrels into a store, &c, are inclined planes. 

182. The advantage gained by the use of the inclin- 
ed plane, like that of the other mechanical powers, is 
gained by a loss of time ; for the weight, instead of 
moving directly up the ascent, must move the whole 
length of the plane. The power of gravity, also, in- 
stead of being confined to the perpendicular height, is 
spread over the whole length of the plane. 

Chisels and other cutting instruments, which are chamfered or 
sloped only on one side, are constructed on the principle oi' the in- 
clined plane. 

183. The wedge consists of two inclined planes unit- 
ed at their bases. 

What does figure 39 represent > For what are cranks often used ? 180. What 
is an inclinedplane? What figure represents an inclined plane; Explain the fig- 
ure. To what is the advantage gained by the use of the inclined plane in pro- 
portion? 181. What follows from the greater or less inclination of the plane? 
Give some instances of the application of the inclined plane. 182. Is any time 
gained by the use of the inclined plane? Upon what principle are chisels and 
other cutting instruments, which are sloped only on one side, constructed » 183 Of 
what does the wedse consist .' 




MECHANICS. 57 

Illustration. Fig. 41 represents a wedge. The line a 
b represents the base of each of the inclined planes of which 
it is composed, and at which they are united. 

The advantage gained by the wedge is in proportion as 
its length exceeds one half its width. 

The wedge is a very important mechanical power, used 
to split rocks, timber, &c, which could not be effected by 
any other power. 
Axes, hatchets, knives, and all other cutting instruments cham- 
fered or sloped on both sides, are constructed on the principle of the 
wedge. 

184. The screw consists of an inclined plane, wound 
round a cylinder. It is generally composed of two 
parts, the screw and the nut; or, as they are generally 
called, the male and the female screw. 

Fig. 42 represents the screw and the nut. S is the 
male screw, (which is an inclined plane wound 
round a cylinder,) N is the nut or female screw 
which has a spiral groove, to which the thread of 
the male screw is accurately fitted. L is a lever 
attached to the uut, to which the power is applied. 
By turning the lever in one di- Fi(Tj 43 

rection the nut ascends, and by L_ 
turning it in the opposite direction 
the nut descends on the screw. * 

In this figure the screw is fixed and the nut is 

moveable. 

Figure 43 represents another screw, which is 

movable. The nut is fixed to the frame, and 

the screw ascends or descends as the lever L is 

turned- 

* Although the screw is mentioned as one of the six mechanical powers, it is, 
in reality, a compound power, consisting of a lever and an inclined plane. The 
power of the screw is estimated by the distance of the threads. The closer the 
threads the greater is the power; but here again the increase of power is procured 
by an increase of velocity, or a loss of time. For if the threads are a quarter of 
an inch apart, the power must move through the whole circumference of the circle 
described by the lever, in ord^r to move the resistance a quarter of an inch. The 
screw, with its appendage the lever, is therefore used for the purpose of moving 
large or heavy bodies through small distances. Its power may be increased by 
lengthening the lever. The screw is applied to presses of all kinds where great 
power is required, such as book-binders' presses, cider and wine presses, &c. 



What does Fig. 41 represent ? To what is the advantage gained by the wedge 
in proportion ? Of what use is the wedge? Give some examples of the wedge. 
184. Of what does the sctew consist? Of how many parts is it generally compos- 
ed.' What are they? What figure represents the screw and the nut i Explain 
the figure. How does figure 43 d iffer from the 42d ? Is the screw a simple or 
compound power? How is the power of the screw estimated? How does the 
closeness of the thread affect the power ? What is the use of the screw ? How 
can its power be increased ? To what is the screw applied? 

6* 





58 NATURAL PHILOSOPHY. 

185. All machines, instruments, implements, &c. are 
composed of one or more of the mechanical powers* 

186. By friction in machinery, is meant the resist- 
ance which bodies meet with in rubbing against each 
other. 

187. There are two kinds of friction, the rolling and 
the sliding. The rolling friction is caused by the roll- 
ing of a circular body. The sliding friction is produced 
by the sliding or dragging of a flat surface. The slid- 
ing friction is overcome with more difficulty than the 
rolling. In calculating the. power of a machine, an al- 
lowance must always be made for friction. It is usual- 
ly computed that friction destroys one third of the pow- 
er of a machine.t 

188. Friction is caused by the unevenness of the sur- 
faces which come into contact ; J and it is diminished in 

* From what has been stated with regard to the mechanical powers, it appears 
that by their aid a man is enabled to perform works to which his unassisted, nat- 
ural strength is wholly inadequate. But the power of all machines is limited by 
the strength ofthe materials of which they are composed, iron, which is the strong- 
est of all substances, will not resist a strain beyond a certain limit. Jts cohesive 
attraction may be destroyed, and it can withstand no resistance which is stronger 
than its cohesive attraction. Besides the strength of the materials, it is necessa- 
ry, also, to consider the time which is expended in (lie application of mechanical 
assistance. Archimedes is said to have boasted to Hiero, ting of Syracuse, that 
if he would give him a place to stand upon lie would move the whole world. In 
order to do this, Archimedes must himself have moved over as much more space 
than he moved the world, as the weight of the world exceeded his own weight ; 
and it has been computed that he must have moved with the velocity of a cannon 
ball for a million of years, in order to move the earth the twenty-seven millionth 
part of an inch. 

f The smallest impediment from friction is when finely polished iron is made 
to rub on bill metal ; but even these are said to lose about one-eighth of their 
moving power. As (lie Iriction between rolling bodies is much less than in those 
that drag, the axie of large wheels is sometimes made to move on small wheels 
or rollers. These are called friction wheels, i»r friction rollers. They turn round 
their own centre as the wheel continues its motion. 

\ All bedies, how well soever they may be polished, have inequalities in their 
surfaces, which may be perceived by a microscope. When, therefore, the surfaces 
of two bodies come into contact, the prominent parts of <he one will often fall into 
the hollow parts of the other, and cause moie or less resistance to motion. 



185. Of what are all machines, instruments., implements, &c. composed .' What 
aid is afforded to man by the use ofthe mechanical powers.' By what is the 
power of all machines limited ? Can the cohesive attraction of iron be destroyed .' 
Can it withstand any resistance stronger than its cohesive attraction? What, be- 
side the strength ofthe material, is necessary to be considered ? What is related ot 
Archimedes? How could Archimedes have done ihis? 186. What is meant by 
friction in machinery ? 187. How many kinds of friction are there! What are 
they? How is the rolling friction produced ? The sliding? Which is overcome 
with the less difficulty, the rolling or sliding? What allowance must always be 
made in calculating the power of a machine ? What proportion ofthe power is 
usually computed to be e'es'royed by friction ? Where is there the least friction >. 
Between which is friction the l^ss, rolling bodies or those that slide? 188. What 
causes friction ? In what proportion is it diminished .' In what manner can it be 
lessened ? 



MECHANICS. 59 

proportion as the surfaces are smooth and well polish- 
ed. Oil, grease, black-lead, or powdered soap-stone is 
used to lessen friction, because they act as a polish by 
filling up the cavities of the rubbing- surfaces, and thus 
making them slide more easily over each other. 

139. Wheels are used on vehicles to diminish the fric- 
tion of the road. The larger the circumference of the 
wheel, the more readily it will overcome any obstacles, 
such as stones or inequalities in the road.* 

190. The motion of all bodies is influenced by the 
medium t in which they move. By a medium is meant 
the substance or fluid which surrounds the body. Thus, 
air is the medium which surrounds a bird when flying; 
water is the medium which surrounds the fish when 
swimming, &c. 

191. The resistance of a medium is in exact pro- 
portion to its density. A body falling through the 
air meets with less resistance than when falling through 
water, because water is a denser medium than air. If 
a machine could be worked in vacuo, (that is, in a va- 
cuum or a space where there is neither air nor anything 
else to impede it) and without friction, it would be per- 
fect. 

192. The main-spring of a watch (See No. 156) con- 
sists of a long ribbon of steel, closely coiled, and con- 
tained in a round box. It is employed instead of a 
weight to keep up the motion. 

As the spring when closely coiled exerts a stronger force than 
when it is partly loosened, in" order to correct this inequality, the 
chain through which it acts, is wound upon an axis surrounded by 

* In descending a steep hill, the wheels of a carriage are often locked, (as it is 
called) that is. fastened in such aman.neras to prevent their turning; and thus the 
rolling h converted into the sliding friction, and the vehicle desceuds more safely. 

Castors are put on the legs of tables and other articles of furniture, to facilitate 
the moving of them ; and thus the sliding is converted into the rolling friction. 

f The plural of this word is media. 

189. What is the use' of wheels ? In what proportion do they overcome the ob- 
stacles, such as stones, &c, in the road ? Why, in descending a steep hill, are the 
wheels of a carriage often locked ! How do casters, which are put upon furniture, 
facilitate the moving of it ! 190. How is the motion of all bodies influenced? 
What is meant by a medium? What is the plural of medium ? ]91. To what is 
the resistance (if a medium in proportion ? What illustration is given ? When 
would a machine he perfect.' 192. Of what does the main-spring of a watch con- 
sist >. What is its use I Does the spring exert a stronger force when closely coiled, 
or when partly loosened ? 



60 



NATURAL PHILOSOPHY. 



a spiral groove, (called a fiisee) gradually increasing in diameter 
from the top to the bottom ; so that in proportion as the strength of 
the spring is diminished it may act on a larger lever, or a larger 
wheel and axis. 
Illustration. Fig. 44 represents a spring coiled in a round box, A. 



Fig. 44. 




B is the fusee surrounded 
by a spiral groove on which 
the chain C is wound. 
When the watch is recent- 
ly wound, the spring is in 
the greatest state of tension, 
and will, therefore, turn 
the fusee by the smallest 
A B groove, on the principle of 

the wheel and axle. As the spring loses its force by being partly 
unwound, it acts upon the larger circles of the fusee ; nd the want 
of strength in the spring is compensated by the mechanical aid of a 
larger wheel and axle in the larger grooves. By this means the 
spring is made at all limes to exert an equal power upon the fusee. 
The motion is communicated from the fusee by a cogged wheel 
which turns with the fusee. 

193. The name of governor has been given to an in- 
genious piece of mechanism which is used to regulate 
the supply of steam in steam-engines, and of water in 
water-mills. 

Illustration. Fig. 45 represents a 
governnor. A B and A C are two 
levers or arms, loaded with heavy balls 
at their extremities, B and C, and sus- 
pended by a joint at A, upon the ex- 
tremity of a revolving shaft, A D. At 
a is a collar, or sliding box, connected 
with the levers by the rods b a and ca, 
with joints at their extremities. When 
the shaft A D revolves rapidly, the 
weights B and C will diverge or fly off, 
and cause the rods b a and c a to raise 
the collar or sliding-box. On the con- 
trary, when the shaft, A D, revolves 
slowly, the weights B and C will fall 
by their own weight, and the rods b a 
and c a will cause the collar a to de- 
The steam-valve in a steam-engine, or the sluice-gate of a 




scend. 



water-wheel, being connected with the collar a, the supply of steam 
or water, which puts the works in motion, is thus regulated.* 

* In manufactures, there is one certain and determinate velocity with which the 
machinery should he moved, and which, if increased or diminished, would render 



What is done in order to correct this inequality ! What does Fig. 44 represent ? 
Explain. 193. What is a governor ; Explain Fig. 45. What is 'said in the note 
of tho use of the governor ? 



HYDROSTATICS. 61 

194. The knee-joint, or as it is sometimes called the 
toggle-joint, consists of two rods or bars connected by a 
joint, and increasing rapidly in power as the two rods 
approach to the direction of a straight line. 

Illustration. Fig. 46 represents a toggle-joint. A C and B C 
are the two rods connected by a joint C. A mov- 
ing force applied in the direction C D acts with 
great and constantly increasing power to sepa- 
rate the parts A and B. 

The operation of the toggle joint is seen in the 
iron joints which are used to uphold the tops of 
chaises. It is also used in various kinds of print- 
ing-presses, to obtain the greatest power at the 
moment of impression. 




SECTION IX. 

Hydrostatics. 

195. Hydrostatics treats of the nature, gravity and 
pressure of fluids. (See No. 5.) 

196. A fluid is a substance which yields to the slight- 
est pressure, and the particles of which, having but a 
slight degree of cohesion, move easily among them- 
selves. (See No. 19.) 

197. A liquid differs from a fluid in its want of com- 
pressibility * and elasticity. (See Numbers 67 andlO.) 

the machine unfit to perform the work it is designed to execute. Now, it frequent- 
ly happens that the resistance is increased or diminished by some of the machines 
which are worked, being stonped, or others put on. The moving power, having 
this alterition in the resistance, would impart a greater or less velocity to the 
machinery, were it not for the regulating power of the governor, which increases 
or diminishes the supply of water or of steam, which is the moving power. 

* The experiments (mentioned in number 26) made at Florence, many years 
ago, seemed to prove that some kinds of Irquids, water, for instance, is wholly ii- 
compressible. Later experiments, particularly those of Mr. Jacob Perkins, of 
Newbury port, now in London, have proved that water is capable of a considerable 
degree of compression. Fluids, in general, have a voluntary tendency to expand 
(See No. 68.) when at liberty; but liquids wHl not expand without a change of 

194. Of what does the knee-joint or toggle-joint consist: In what proportion 
does it increase in power ! What doe/tigure 4i> represent > Explain the figure. 
Give an instance of the operation of the toggle-joint? What is its use in print- 
ing-presses' 195. Of what does Hydrostatics treat i J9»>- What is a fluid? 
Does the attraction of cohesion have much influence on the particles of fluids? 
What follows from this ? 197. [low do fluids and liquids differ from each other ? 
Can water be compressed i What is supposed to be the primary cause of the flu- 
id form of bodies ? What effect has heat upon bodies ! What illustration is given s 



62 NATURAL PHILOSOPHY. 

198. Fluids gravitate in a more perfect manner than 
solids, because the strong cohesion of the particles of 
solid hodies in some measure counteracts the effects of 
gravity. Thus every particle of a fluid, which is not 
supported on all sides, will fall; but the cohesive attrac- 
tion of the particles of solids enables the legs of a ta- 
ble to support a considerable weight. From this cir- 
cumstance it appears that fluids have only a slight de- 
gree of cohesive attraction. 

199. From the slight degree of cohesion in the par- 
ticles of fluids it is inferred that they must be small, 
smooth and globular ; smooth, because there appears 
to be no friction among them ; and globular because 
touching each other but by a point would account for 
the slightness of their cohesion. 

200. Fluids cannot be formed into figures, or pre- 
served in heaps on account of their want of cohesion. 

201. By the level or equilibrium of fluids is meant 
that every part of the surface is equally distant from 
the centre of the earth ; that is, from the point to which 
gravity tends. 

Illustration. All fluids have a tendency to preserve this equi- 
librium. Hence the surface of all fluids when in a state of rest 
must partake of the spherical form of the earth, and will therefore 
be bulging, not flat. This is very evident in large bodies of water, 
such as the ocean ; and causes the masts of vessels at a distance to 
be seen before the hull. But the surfaces of small bodies of water 
bulge so little that they appear flat. This level or equilibrium of 
fluids is the natural result of the independent gravitation of each 
particle. The particles of a solid body being united by cohesive at- 
traction, if any one of them is supported, it will uphold those also 
with which it is united. But when any particle ot a fluidU unsup- 
ported, it is attracted down to the level of the surface of the fluid; 
and the readiness with which fluids yield to the slightest pressure 

temperature. Heat is supposed to be the primary cause of Ihe fluid form of bod- 
ies. (SbcJVo. 61.) It insinuates itself between the particles of bodies, and forces 
them asunder. Thus, for instance, ice, without heat, is a solid ; with heat it be- 
comes water, and with a greater degree of heat it expauds into an elastic fluid 
called steam. 

198. Why do fluids gravitate in a more perfect manner than solids! How 
would you prove that fluids have only a slight degree of cohtsive attraction? 
199. What is inferred from the slight degree of cohesion in the particles of 
fluids! Why smooth ? Why globular! 200. Why cannot fluids be formed 
into figures, or preserved in heaps ! 201. What is meant by the level or equilibri- 
um of fluids' Have all fluids a tendency to preserve this equilibrium ! What 
follows from this ! W T hy do some surfaces appear flat ! Of what is this level or 
equilibrium of fluids the natural result! How does tho gravitation of solid 
bodies differ from that of fluids ! 



HYDROSTATICS. 63 

will enable the particle, by its own weight, to penetrate the surface 
of the fluid and mix with it. 

202. Fluids of different densities all preserve their 
own equilibrium. 

Illustration. If a quantity of mercury, water, oil and air, be put 
into the same vessel, they will arrange themselves in the order of 
their specific gravities. (See No. 84.) The mercury will sink to 
the bottom, the water will stand above the mercury, the oil above 
the water, and the air above the oil; and the upper and under sur- 
faces of each fluid will partake of the spherical form of the earth, 
to which they all respectively gravitate. 

203. A water-level is an instrument constructed on 
the principle of the equilibrium of fluids. It consists 
of a glass tube, partly filled with water, and closed at 
both ends. When the tube is not perfectly horizontal 
— that is, if one end of the tube be lower than the oth- 
er — the water will run to the lower end. By this 
means the level of any situation to which the instru- 
ment is applied may be ascertained. 

Illustration. Fig. 47 represents a water-level. A B is a glass 
Fig. 47. tube partly filled with water. C is a bubble 

A c B of air occupying the space not filled by the 

i — v ,_, y — , water. When both ends of the tube are on 
^ P a level, the air bubble will remain in the cen- 

tre of the tube ; but if either end of the tube be depressed, the wa- 
ter will descend and the air bubble will rise. The glass tube when 
used is generally set in a wooden or brass box. It is an instrument 
much used by carpenters, masons, surveyors, &c. 

204. The inertia (See No. 39.) of fluids is consider- 
ably less than that of solid bodies ; because the strong 
cohesion of the particles of solid bodies causes them 
unitedly to resist every change of state, whether of 
motion or rest; but the resistance of the particles of 
fluids may be more easily overcome, on account of their 
want of cohesion, which prevents them from acting to- 
gether. Solid bodies, therefore, gravitate in masses — 
their parts being so connected as to form a whole, their 
weight is concentrated in a single point called the cen- 



202. Do fluids of different densities all preserve their own equilibrium ? What 
illustration is given to prove this ! 203. Upon what principle is a water level 
constructed .» Of what does it consist ? For what is it used I What figure rep- 
represents a water-level ! Explain the figure. 204. How does the inertia of flu- 
ids compare with that, of solid bodies? Why! In what manner do solid bodies 
gravitate I What is the centre of gravity .' 



64 NATURAL PHILOSOPHY. 

tre of gravity ; while every particle of a fluid may be 
considered as a separate mass, gravitating indepen- 
dently of each other. It is for this reason that a body 
of water, in falling, does less injury than a solid body 
of the same weight. But if the water be converted 
into ice, the particles losing their fluid form, and being 
united by cohesive attraction, gravitate unitedly in one 
mass. 

205. The effect of gravity on the particles of fluids 
is peculiar. It causes them not only to press down- 
wards like solids, but also upwards, sideways, and in 
every direction. So long as the equality of pressure 
is undisturbed, every particle will remain at rest. If 
the fluid be disturbed by agitating it, the equality of 
pressure will be disturbed, and the fluid will not rest 
until the equilibrium is restored. 

Illustration. The downward pressure of fluids is shown by mak- 
ing an aperture in the bottom of a vessel of water. Every particle 
of the fluid above the aperture will run downwards through the 
opening. 

The lateral pressure is shown by making the aperture at the side 
of the vessel. The fluid will then escape through the aperture at 
the side. 

The upward pressure is shown by taking a glass tube, open at 
both ends, putting a cork into one end, (or stopping it up with the 
finger) and immersing the other in the water. The water will not 
rise in the tube. But the moment that the cork is taken out, (or the 
finger is removed,) the fluid will rise in the tube to a level with the 
surrounding water. 

208. The particles of fluids are not arranged in reg- 
ular columns, one above another, for if they were, there 
would be no lateral pressure. 

Illustration- Fig. 48 represents the magnified particles of a fluid 
Fj... 8. arranged in regular columns. It is evident, from an 

inspection of the figure, that the effect of gravity 
upon each particle w ? ill be to carry it downward only 
by a force equal to its own weight, added to the 
weight of each particle above it. Fig. 49. Fij. 49. 
represents the manner in which the parti- 
cles are probably arranged, and it appears 
by that figure that each particle presses between two par- 
ticles beneath it, and that these last must suffer a lateral 
pressure. 

20.5. What effect has gravity on the particles of fluids? How long will th« 
particles of fluids remain at rest? How is the downward pressure of fluids shown .' 
The lateral pressure.' The upward pressure? 206. Are the paitichs of fluidg 
arranged in regular columns, one above another ? What would be the consequence 
if they were .' Explain Fig. 48. What does i figure 49 represent I 




HYDROSTATICS. 65 

207. The pressure of a fluid is in proportion to the 
perpendicular distance from the surface ; that is, the 
deeper the fluid the greater will be the pressure. This 
pressure is exerted in every direction, so that all the 
parts at the same depth press each other with equal 
force. 

Illustrations. A bladder, filled with air, being immersed in wa- 
ter, will be contracted in size, on account of the pressure of the wa- 
ter in all directions ; and the deeper it is immersed the more will it 
be contracted. 

An empty bottle, being corked, and by means of a weight let 
down to a certain depth in the sea, will either be broken by the pres- 
sure, or the cork will be driven into it, and the bottle be filled with 
water. This will take place even if the cork be fastened with wire 
and sealed. But a bottle filled with water, or any other liquid, may- 
be let down to any depth without damage, because, in this case, the 
internal pressure is equal to the external.* 

208. From what has now been stated, it appears that 
the lateral pressure proceeds entirely from the pressure 
downwards, or, in other words, from the weight of the 
liquid above ; and that consequently the lower an ori- 
fice is made in a vessel containing water or any other 
liquid, the greater will be the force and velocity with 
which the liquid will rush out. 

* ' Experiments at Sea. — We aro indebted to a friend, who has just arrived from 
Europe, says the Baltimore Gazette, for the following experiments made on board 
the Charlemagne: 

'26th of September, 1836, the weather being calm, I corked an empty wine bottle 
and lied a piece of linen over the cork; I then sank it into the sea six hundred 
feet ; when drawn immediately up again, the cork was inside, the linen remained 
as it was placed, and the bottle was filled with water. 

' I next made a noose of strong twine around the bottom of the cork, which I 
forced into the empty bottle, lashed the twine securely to the neck of the bottle, 
and sank the bottle six hundred feet. Upon drawing it up immediately the cork 
was found inside, having forced its way by the twine, and in so doing had broken 
itself in two pieces ; the bottle was filled with water. 

' I then made a stopper of white pine, long enough to reach to the bottom of the 
bottle; after forcing this stopper into the bottle, I cut it off about half an inch 
above the top of the. bottle and drove two wedges, of the same woodj into the 
stopper. I sank it 600 feet, and upon drawing it up immediately the stopper re- 
mained as I placed it, and there was about a gill of water in the bottle, which re- 
mained unbroken. The water must have forced its way through the pores of the 



207. To what is the pressure of a fluid in proportion? In what direction is 
this pressure exerted? What illustrations are given to prove this? Why can a 
bottle, filled with water, or any other liquid, be let down to any depth without in- 
jury .' What experiment is mentioned in the note? 208. What causes the late- 
rat pressure I What follows from this .' 




66 NATURAL PHILOSOPHY. 

F, S- 50 * Illustration. Fig. 50 represents a vessel 

of water, with orifices at the side at different 
distances from the surface. The different 
curves in that figure, describing the course 
of the liquid in running out of the vessel, 
show the force of the pressure on the liquid 
at different depths. At A the pressure is the 
least, because there is less weight of fluid 

above it. At B and C the fluid is driven downwards by the weight 

of that portion above, and it will be strongest at C. 

209. As the lateral pressure arises solely from the 
downward pressure, it is not affected by the width or 
the length of the vessel in which it is contained, but 
merely by its depth; for as every particle acts inde- 
pendently of the rest, it is only the column of parti- 
cles immediately above the orifice that can weigh upon 
and press out the water. 

210. The lateral pressure on one side of a cubical 
vessel will he equal only to half of the pressure down- 
wards ; for every particle at the bottom of the vessel 
is pressed upon by a column of the whole depth of the 
fluid, whilst the lateral pressure diminishes from the 
bottom upwards to the surface, where the particles have 
no pressure. 

211. The upward pressure of fluids, although appa- 
rently in opposition to the principles of gravity, is but 
a necessary consequence of the operation of that prin- 
ciple ; or, in other words, the pressure upwards, as well 
as the pressure downwards is caused by gravity. 

Illustration. When water is poured into a vessel with a spout 
(like a tea-pot, for instance,) the water rises in the spout to a level 
with that in the body of the vessel. The particles of water at the 
bottom of the vessel, are pressed upon by the particles above them, 
and to this pressure they will yield, if there is any mode of making 

wooden stopper, although wedged as aforesaid ; and had the bottle remained sunk 
lonjf enough, ihere is no doubt that it would have been filled with water.' 

It is the opinion of some philosophers that the pressure at very great depths of 
the sea is so great that the water is condensed into a solid slate; and that at or 
near the centre of the earth this pressure converts the whole into a solid mass of 
fire. 

Explain Fig. 50. 209. Does the length or the v/idth of the vessel in which it is 
contained have any effect upon the lateral pressure ? By what is it affected ? 
210. How does the lateral pressure on one side of a cubical vessel compare with 
the pressure downward ? How would you explain this? 211. What causes the- 
upward and downward pressures-' 



HYDROSTATICS. 



67 




way for the particles above them. As they cannot descend through 
Fig. 51. the bottom of the vessel they will change their 

direction and rise in the spout. Fig. 51 repre- 
sents a tea-pot, and the columns of balls repre- 
sent the particles of water magnified. From 
an inspection of the figure it appears that the 
particle numbered 1, at the bottom, willbe press- 
ed laterally by the particle numbered 2, and by 
this pressure forced into the spout, where meet- 
ing with the particle 3 it presses it upwards, and this pressure will 
be continued from 3 to 4, from 4 to 5, and so on till the water in the 
spout has risen to a level with that in the body of the vessel. If wa- 
ter be poured into the spout the water will rise in the same manner 
in the body of the vessel ; from which it appears that the force of 
pressure depends entirely on the height, and not on the length or 
breadth of the column of fluid, as is stated in No. 209. 

The Hydrostatic Bellows. From what has now been stated it appears 
that any quantity of fluid, however small, may be made to counter- 
poise or balance any quantity, however large. This is called the 
hydrostatical paradox, and it is shown by an instrument called the 
hydrostatical bellows. 

Fig. 52. 

Fig. 52 represents the hydrostatic bel- 
lows. A B is a long tube, one inch square. 
C D E F are the bellows, consisting of two 
boards, eight inches square, connected by 
broad pieces of leather, or India rubber 
cloth, in the manner of a pair of common 
bellows. By putting one pound of wa- 
ter in the tube, it will raise sixty -four 
pounds on the bellows/ The Hydrostatic 
Bellows belonging to " the Boston School 
set " are eight inches square, marked into 
sixty four square inches, on the top — or in- 
to sixteen squares of two inches each. There 
are two square tubes connected with the 
bellows, one of one inch and another of two 
inches in diameter, or a sixty-four and a 
sixteenth of the surface of the bellows. If 
a pound of water be put into the larger tube, sixteen pounds may be 
raised on the bellows, but if it be put into the smaller tube it will 
raise sixty-four pounds. 

* The fundamental principle of mechanics or the laws of motion is here also 
in full force, namely, that what is gained in power is lost either in time or in 
space ; for although one pound is here made to raise sixty-four pounds, it is to be 
remarked that the distance or height to which the sixty-four pounds will rise is as 
much less than that over which the one pound will move, as sixty-four is greater 
than one. 




Illustrate this by figure 51. Upon what does the force cf pressure depend? 
What is meant by the hydrostatic paradox ? What is the use of the hydrostatic 
bellows? What Fig. represents the hydrostatic bellows? Explain the Fig. 
What is the fundamental principle of mechanics ? Is this the principle of 
the hydrostatic bellows? 



68 



NATURAL PHILOSOPHY. 



When the bellows have been filled with water, turn the stop cock; 
take out the tube and substitute the straight jet, (See rig. 72.) and 
the water will be forced out to a height nearly as great as that of 
the water in the tube. Were it not for the resistance of the air it 
would rise to the same height. 

212. If water be confined in any vessel, and a pres- 
sure to any amount be exerted on a square inch of that 
water, a pressure to an equal amount will be transmit- 
ted to every square inch of the surface of the vessel in 
which the water is confined.* 

Fig. 53. Illustration. It is upon 

this principle that Bra- 
mah's hydrostatic press, 
represented in Fig. 53. is 
constructed. A large sol- 
id plug or piston, A B, is 
constructed so as to move 
water-tight in a cylinder 
C D. The space beneath 
the piston is filled with 
water, and communicates 
by a pipe E F with a 
small forcing-pump work- 
ed by the piston G, by 
which the water is forc- 
ed into the chamber of the cylinder C D below the great piston. 
Let us now suppose the entire space between the two pistons to be 
filled with water, and a pressure of one pound exerted on the water 
by means of the piston G of the forcing-pump. Let us also suppose 
that the diameter of the piston G is a quarter of an inch, and that 
the diameter of the piston B is one foot. In that case, the base of 
the piston B, which is pressed by the water, is 2304 times the base 
of the piston G, which presses the water, and. in virtue of the pow- 
er of transmitting pressure, a pressure of one pound will be trans- 
mitted to every part of the base of the greater piston, which is equal 
to the base of the less. Thus an urging pressure of one pound on 
the base of the piston G, will produce a pressure of 2304$s. 
against the base of the greater piston B. 

*This property of fluids, therefore, seems to invest us with a power of increasing 
the intensity of a pressure exerted by a comparatively small force, without any- 
other limit than that of the strength of the materials of which the engine itself is 
constructed. It also enables us with great facility to transmit the motion and 
force of one machine to another, in cases where local circumstances preclude the 
possibility of instituting any ordinary mechanical connexion between the two ma- 
chines. Thus, merely by means of water-pipes, the force of a machine may be 
transmitted to any distance, and over inequalities of ground, or through any oth- 
er obstructions. 




212. What fact is mentioned in this number with regard to the pressure on 
water? Upon what principle is Bramah's hydrostatic press constructed? What 
Fig. represents this > Explain the Fig. What advantages result from that prop- 
erty of fluids stated in No. 212 .' 



HYDROSTATICS. 69 

213. A fluid specifically lighter than another fluid 
will float upon its surface.* 

214. A body specifically lighter than a fluid will sink 
in the fluid until it has displaced a portion of the fluid 
equal in weight to itself. 

Illustration. If a piece of cork is placed in a vessel of water, 
about one third part of the cork will sink below, and the remainder 
will stand above the surface of the water; thereby displacing a por- 
tion of water equal in bulk to about a third part of the cork, and 
this quantity of water is equal in weight to the whole of the cork; 
because the specific gravity (See No. 84.) of water is about three 
times as great as that of cork, t 

215. The standard which has been adopted to esti- 
mate the specific gravity (See No. 84.) of substances 
in general, is rain or distilled water. 

Explanation. As heat expands and cold condenses all metals, 
their specific gravity cannot be the same in summer that it is 
in winter. For this reason they will not serve as a stand- 
ard to estimate the specific gravity of other bodies. The reason 
that distilled water is used is, that spring, well, or river water 
is seldom perfectly pure; and the various substances mixed 
with it affect its weight. But distilled water is uniformly of the 
same weight. Taking, therefore, a certain quantity of rain or dis- 
tilled water, we find that a quantity of gold, equal in bulk to the wa- 
ter, will weigh nearly twenty times as much as the water; of lead, 
nearly twelve times as much; while oil, spirit, cork, &c. will weigh 
less than the water, t 

* The slaves in the West Indies, it is said, steal rum by inserting the long neck 
of a bottle, full of water, through the top aperture of the rum cask. The water 
falls out of the bottle into the cask, while the lighter rum ascends in its stead. 

f It is on the same principle that boats, ships, &c. although composed of mate- 
rials heavier than water, are made to float. From their peculiar shapo they are 
made to set lightly on the water. The extent of the surface presented to the 
water counterbalances the weight of the materials, and the vessel sinks to such a 
depth as will cause it to displace a portion of water equal in weight to the 
whole weight of the vessel. From a knowledge of the specific gravity of water, 
and the materials of which a vessel is composed, rules have been formed by which 
to estimate the tonnage of vessels— that is to say, the weight which the vessel will 
sustain without sinking. 

X The following table shows the specific gravity of the substances therein men- 
tioned. It is to be understood that all substances whose specific gravity is greater 
than water, will sink when immersed in it, and that all whose specific gravity is 
less than that of water, will float in it. Let us then take a quantity of water 



213. When will one fluid float upon another ? 214. What is stated with re- 
gard to a body specifically lighter than a fluid ? What illustration of this is 
given? How do the specific gravities of water and cork compare with each oth- 
er ? Upon what principle is it that boats, ships, &c. are made to float upon the 
water ? What rules have been formed from the knowledge of the specific gravity 
of water and the materials of which vessels are composed! 215. What standard 
has been adopted to estimate the specific gravity of substances in general ! Why 
could not metals have been adopted ? Why is distilled water used .' 




70 NATURAL PHILOSOPHY. 

216. The specific gravity of bodies that will sink in 
water is ascertained by weighing them first in water, 
and then out of the water, and dividing the weight out 
of the water by the loss of weight in water. 

Fig. 54. Fig. 54 represents the scales for ascertain- 

ing the specific gravity of bodies. One scale 
is shorter than the other, and a hook is at- 
tached to the bottom of the scale to which 
substances, whose specific gravity is sought 
may be attached and sunk in water. 

Illustration. Suppose a cubic inch of gold 
weighs 19 ounces when weighed out of the 
water, and but 18 ounces * when weighed 
in water — the loss in water is one ounce. 
The weight out of water, 19 ounces, being 
divided by one (the loss in water) gives 19. 

The specific gravity- of gold, then, would be 19, or, in other words. 

gold is nineteen times heavier than water. 

which will weigh exactly one pound ; a quantity of the substances specified in the 

table, of the same bulk, will weigh as follows : 

Piati:ium, 23. pounds. "Chalk, 

Fine Gold, 19.640 • Coal, 

Mercury, 14.019 ' Mahogany, 

Lead, 11.593 ' Milk, 

Silver, 11-091 ' Box wood, 

Copper, 9.000 ' Rain water, 

Iron 7.645 ' Oil, 

Marble, 2.705 ' Ice, 

Glass, 3.000 ' Brandy, -820 

A cubic foot of water weighs one thousand avoirdupois ounces. By multiplying 
the number opposite to any article in the above table by one thousand, we obtain 
the weight of a cubic foot of that article, in ounces. Thus a cubic foot of plati- 
num is 23000 ounces in weight. 

In the above table it appears that the specific gravity of living men is about 
one ninth less than that of common water. So long, therefore, as the lungs can 
be kept free from water, a person, although unacquainted with the art of swim- 
ming, will n*»t completely sink, provided the hands and arms be kept under the 
water. 

The specific gravity of sea water is greater than that of the water of lakes and 
rivers, on account of the salt contained in it. On this account the water of lakes 
and rivers has less buoyancy, and it is more difficult to swim in it. 

* Gold will weigh less in the water than out of it, on account of the upward 
pressure of the particles of water, which in some measure supports the gold, and 
by so doing diminishes its weight. Now, as the upward pressure of these parti- 
cles is exactly sufficient to balance the downward pressure of a quantity of water 
of exactlv the same dimensions with the gold, it follows that the gold" will lose 
exactly as much of its weight in water as a quantity of water of the same di- 

What bodies will sink when immersed in water? What will float r What is 
the weight of a cubic foot of water ! What is the use of the above table? How 
does the specific gravity of living men compare with that of water? Which is 
the greater, the specific gravity of sea water, or of lakes and rivers ? Why I 216. 
How is the specific gravity of bodies, that will sink in water, ascertained ! What 
illustration is given ? Explain Fig. 54. Why will gold weigh less in the water than 
out of it! How does this upward pressure of the particles compare with the 
downward pressure of a quantity of water of the same dimensions ? What fol- 
lows from this ? 



1.793 pounds. Living men, 


.891 pounds 


1.250 


Ash, 




.800 ' 


1.053 


Beach, 




.700 ' 


1.034 


Eim, 




•600 « 


1.030 


Fir, 




.500 ' 


1.000 ' 


Cork, 




.210 l 


.920 


C mmon 


Air, 


.0011 ' 


.9J8 « 


H^drogei 


g^, 


.000105 < 



HYDROSTATICS. 71 

217. The specific gravity of a body that will not 
sink in water, is ascertainad by dividing its weight, by 
the sum of its weight, added to the loss of weight 
which it occasions in a heavy body previously balanced 
in water.* 

Illustration. If a body lighter than water weighs six ounces, and 
on being attached to a heavy body, balanced in water, is found to 
occasion it to lose twelve ounces of its weight, its specific gravity 
is determined by dividing its weight (six ounces) by the sum of its 
weight, added to the loss of weight it occasions in the heavy body, 
namely, six added to twelve, which, in other words, is G divided by 
18, or 6-18, which is l-3d. 

218. An hydrometer is an instrument to ascertain 
the specific gravity of liquids. 

Illustration. The hydrometer is constructed on the principle, 
that the greater the weight of a liquid, the greater will be its buoy- 
ancy. The hydrometer is made in a variety of forms, but it m gen- 
eral consists of a hollow ball of silver, glass, or other material, 

dimensions with the gold will weigh. And this rule applies to all bodies 
heavier than water, that are immersed in it. They will lose as much of 
their weight in water as a quantity of water of their own dimensions weighs. All 
bodies, therefore, of the same size, lose the same quantity of their weight in wa- 
ter. Hence, the specific «ravity of a body is the weight of a body, compared with 
that of water. As a body loses a quantity of its weight when immersed in water, 
it follows that when the body is lifted from the water, that portion of its weight 
which it had lost will be restored. This is the reason that a bucket of water, 
drawn from a well, is heavier when it rises above the surface of the water in the 
well, than it is while it remains below the surface. For the same reason our 
limbs feel heavy in leaving a bath. 

* The method of ascertaining the specific gravities of bodies was discovered ac- 
cidentally by Archimedes. He had been employed by the king of Syracuse to in- 
vestigate the metals of a golden crown which he suspected had been adulterated 
by the workmen. The philosopher labored at the problem in vain, till going one 
day into the bath, he perceived that the water rose in the bath in proportion to 
the bulk of his body. He instantly perceived that any other substance of equal 
size would raise the water just as much, though one of equal weight and less 
bulk could not produce the same effect. He then obtained two masses, one of 
gold and one of silver, each equal in weight to the crown, and having filled a 
vessel very accurately with water, he first plunged the silver mass into it, and ob- 
served the quantity of water that flowed over; he then did the same with the 
gold, and found that a less quantity had passed over than before. Hence he in- 
ferred that, though of equal weight, the bulk of the silver was greater than that 
of the gold, and that the quantity of water displaced was, in each experiment, 
equal to the bulk of the metal. He next made trial with the crown, and found it 
displaced more water than the gold, and less than the silver, which led him to 
conclude, that it was neither pure gold nor pure silver. 

What rule is given with regard to all bodies heavier than water that are im- 
mersed in it ? What is the specific gravity of a body ? What is the reason that a 
bucket of water, drawn from a well, is heavier when it rises above the surface of 
the water than while it is below it ? 217. How can the specific gravity of bod- 
ies that will not sink in water be ascertained? What illustration is given? By 
whom was the method of ascertaining the specific gravities of bodies discovered? 
In what manner did he ascertain it ? 218. What is an hydrometer ? Upon what 
principle is it constructed ? Explain its construction. 



72 NATURAL PHILOSOPHY. 

with a graduated scale rising from the upper part. A weight is at- 
tached below the ball. When the instrument, thus constructed, is 
immersed in a fluid, the specific gravity of the fluid is estimated by 
the portion of the scale that remains above the surface of the fluid. 
The greater the specific gravity of the fluid the less will the scale 
sink. 



SECTION X. 

Hydraulics. 

219. Hydraulics treats of the motion of fluids, par- 
ticularly of water ; and the construction of all kinds 
of instruments and machines for moving them.* 

220. Water, in its motion, is retarded by the friction 
of the bottom and sides of the vessel or channel through 
which it passes. For this reason the velocity of the 

* In the second illustration, under No. 38, page 12, some account is given of 
the chemical action of heat upon water ; and the reason is there given why the 
rain which falls upon the earth, and sinks into it, does not, in the course of time, 
injure its solidity. The cause of the ascent of steam, or vapor, may be found in 
its specific gravity. It may here be stated that rain, snow and hail are formed by 
the condensation of the particles of vapor in the upper regions of the atmosphere. 
The watery panicles coming within the sphere of each other's attractions, unite 
in the form of a drop, which being heavier than the air, falls to the earth. Snow 
and hail differ from rain only in the different degrees of temperature at which the 
particles unite. When rain, snow, or hail fall, part of it reascends in the form 
of vapor, to form clouds, &c, partis absorbed by the roots of vegetables, and 
part descends into the earth to form springs. The springs form brooks, rivulets, 
rivers, &c. and descend to^the' ocean, where, being again heated by the sun, the wa- 
ter risesjin the form.of vapor, again forms clouds, and again descends in rain, snow, 
hail. &c. The specific gravity of the watery particles which constitute vapor, is 
le3s than that of the air near the surface of the earth ; they will, therefore, as- 
cend until they reach a portion of the atmosphere of the same specific gravity with 
themselves. But the constant accession of fresh vapor from the earth, and the 
loss of heat, causes several particles to come within the sphere of each other's at- 
traction, as has been stated above, and they unite in the form of a drop, the spe- 
cific gravity of which being greater than that of the atmosphere, it will fall in the 
form of rain. Water, as it descends in rain, snow, or hail, is perfectly pure, but 
when it has fallen to the earth, it mixes with the various substances through 
which it passes, which give it a species of flavor, without affecting its trans- 
parency. 

In what proportion does the scale sink? 219. Of what does hydraulics treat ? 
What is the cause of the ascent of steam or vapor? How are the particles of 
this vapor formed into rain, snow, or hail ? How long will these particles remain 
in the upper regions ? What becomes of them after they have fallen ? 220. What 
retards trie motion of water ? 



HYDRAULICS. 



surface of a canal or river is always greater than that 
of any other part.* 

221. A fluid running from an orifice in a vessel is 
discharged with double the rapidity when the vessel 
from which it flows is kept constantly full. 

222. When a fluid spouts from several orifices in the 
side of a vessel, it is thrown to the greatest distance 
from the orifice nearest to the centre.! 

223. A vessel filled with any liquid will discharge a 
greater quantity of the liquid through an orifice to 
which a short pipe is fitted, than through an orifice of 
the same size without a pipe.| If the pipe projects in- 
to the vessel the quantity discharged will be diminished 
instead of increased by the pipe. 

224. The quantity of a fluid discharged through a 
pipe or an orifice is increased by heating the liquid ; 
because heat diminishes the cohesion of the particles, 
which exists, to a certain degree, in all liquids. 

225. The velocity of a current of water may be as- 
certained by immersing in it a bent tube, shaped like a 
tunnel at the end which is immersed. 



* In eonsequence of the friction of the banks and beds of rivers, and the nu- 
merous obstacles they meet in their circuitous course, their progress is slow. If it 
were not for these impediments, the velocity which the waters would acquire 
would produce very disastrous consequences. An inclination of three inches in a 
mile, in the bed of a river, will give the current a velocity of about three miles an 
hour. 

f If the vessel be elevated, the lowest orifice will discharge the fluid to the 
greatest distance, but when the vessel is placed low, the fluid will reach the plane 
before its projectile force is expended. [See No. 198.] 

J. This is caused by the cross currents m:idc by the rushing of the water from 
different directions towards the sharp-edged orifice. The pipe smooths the pas- 
gage of the liquid. 



Why does the surface of a canal or river have a greater velocity than any other 
part? What benefit results from friction retarding the motion of watei? 221. 
Does the fulness of a vessel from the orifice of which a fluid is running, have any 
effect upon its velocity ? 222. When a fluid spouts from several orifices in tho 
Fide of a vessel, from which is it thrown to the greatest distance ? If the vessel 
be elevated, from which will it be discharged to the greatest distance? Why will 
not this be the case when the vessel is placed low? "223. What effect will a' pipe, 
fitted to an orifice, have with regard to the quantity discharged ? What will be 
the effect if the pipe project into the vessel? How is this caused? 224. How 
can the quantity discharged through a pipe or orifice, be increased ? Why will 
heat increase it ? 225. How can the velocity of a current of water be ascertained ? 



74 



NATURAL PHILOSOPHY. 



Fig. 55. 



a 




Illustration. Fig. 55 is a tube shaped 
like a tunnel, with the larger end im- 
mersed in an opposite direction to the 
current. The rapidity of the current 
is estimated bv the height to which the 
water is forced into the tube, above the 
surface of the current. By such an 
instrument the comparative velocity of 
different streams, or the same stream 
at different times, maybe estimated. 

226. Waves are caused by 
the friction between air and 
water.* 

227. The instruments used 
for raising or drawing water or 
other liquids, are the syphon, the 

common pump,t the chain pump, the forcing pump, 

and the screw of Archimedes. 

228. The screw of Archimedes is a machine said to 

have been invented by the philosopher Archimedes, for 

raising water and draining the lands of Egypt, about 

200 years before the Christian era. 

Fig- 56. illustration. Fig- 

ure 56 represents the 
screw of Archime- 
des. A single tube, 
or two tubes, are 
wound in the form 
of a screw around a 
shaft or cylinder, 
supported by the 
prop and the pivot 
A, and turned by the 
handle, n. As the 
end of the tube dips 
into the water, it is 
filled with the fluid, 

* Ft has been said, (and the experiment has been tried,) that when oil is poured 
on the windward side of a pond, the whole surface will become smooth. The oil 
protects the water from the friction of the wind or air. It is said, also, that 
boats have been preserved in a raging surf, in consequence of the sailors having 
emptied a barrel of oil on the water, which has thus been protected from the fric- 
tion of the air. 

r The common pump, and the forcing pump will be explained in connexion with 
pneumatics. 

What does Fig. 55 represent ? How is the rapidity of the current estimated ? 
What is the use of the instrument ? What causes waves ? What is sometime' 
done to remove this friction ? 227. What instruments are used for raising li- 
quids? 228. What is said of the screw of Archimedes? Explain the use of this 
screw by Fig. 56. 




HYDRAULICS. /5 

,nch is forced up ihe tube by every successive revolution, until it 
is discharged at the upper end. 

229. The chain pump is a kind of pump used on 
board of ships. 

F»g- 57 « Illustration. Fig. 57 represents a chain 

pump. It consists of a square box through 
which a number of square boards or buckets, 
connected by a chain, are made to pass. The 
chain passes over the wheel C and under the 
wheel D, which is under water. The buck- 
ets are made to fit the box, but not so as to 
create much friction. The upper wheel, C, 
is turned by a crank, (not represented in the 
Fig.) which causes the chain with the buckets 
attached to pass through the box. Each buck- 
et, as it enters the box, lifts up the water above 
it, and discharges it at the top. 



230. Springs and rivulets are form- 
ed by the water, from rain, snow, &c. 
which penetrates the earth, and de- 
scends until it meets a substance 
which it cannot penetrate. A reser- 
voir is then formed by the union of 
small streams under ground, and the 
water continues to accumulate until 
it finds an outlet. 




=#i 



Illustration. Fig. 58 Fig. 58. 

represents a body of wa- 
ter, A, formed* by the 
continual accession of 
water received from the 
ducts or rivulets, aaaa. 
When the water rises as 
high as B it finds a pas- 
sage out of the cavity, 
and runs on till it makes 
its way out of the ground 
at the side of a hill, and 
then forms a spring at C. 

231. A spring will 
rise higher than the reservoir from whence it issues. 




rise nearly as high, but cannot 



229. Where is the chain pump used ? What Fig. represents it ? Explain the 
Fig. 230. How are springs and rivulets formed ? Explain Fig. 58. 



76 



NATURAL PHILOSOPHY. 



Fig. 59. 



Water may be conveyed over hills and valleys in bent 
pipes and tubes, or through natural passages, to any 
height which is not greater than the level of the reser- 
voir from whence it flows.* 

232. Fountains are formed .by water carried through 
natural or artificial ducts from a reservoir. The water 
will spout through the ducts to nearly f the height of 
the surface of the reservoir. 

233. The Syphon i; is a tube bent in the form of the 
letter U, one side being a little longer than the other. 

Illustration. Fig. 58 represents the Syphon of the Boston School 
set. A syphon is used by filling it with water or some other fluid, 
then stopping both ends, and in this state immersing the shorter leg 
or side into a vessel containing a liquid. The ends then being un- 
stopped, the liquid will run through the syphon until 
the vessel is emptied. In performing this experiment, 
the end of the syphon which is out of the water must 
always be below the surface of the water in the vessel. 
The syphon may be used to show the equilibrium of 
fluids, by pouring in a small quantity of mercury and 
thirteen and a half inches of water into the largest 
part. The liquids will rise in each side or leg of 
the syphon, in height, proportioned to their specific 
gravity. The mercury being of specific gravity 
thirteen times greater than that of water, will bal- 
ance thirteen times its bulk of water. Consequent- 
I J J ly the water will rise thirteen times as high on one side 
VV^y/ of the syphon as the mercury is on the other. But if 
one liquid only is poured into the syphon it will rise to 
the same height in both sides or leg? of the syphon. Any other li- 
quids may be used with similar effect ; namely, the lighter liquid 
will rise as much higher on one side of the syphon than the other as 
the specific gravity of one fluid exceeds that of the other. 

* The ancient Romans, ignorant of this properly of fluids, constructed vast 
aqueducts across valleys, at tjseat expense, to convey water over them. The 
moderns effect the same object by means of- wooden, metallic or stone pipes. 

f The resistance of the air prevents the fluids from rising to quite the same 
height with the reservoir. 

X The Syphon belonging to " the Boston School set " is a glass tube, the longer 
arm of which is about 6, and the shorter arm about 21 inches in length. Besides 
the experiments made with it, which are mentioned above, the following may be 
performed. I. Screw the stop cock ( See Fig. 66.) into the short end of the sy- 
phon ; close the stop cock, and pour a quantity of mercury into the longest arm. 
The air contained in the shorter arm will prevent the mercury from rising in that 
arm, but on turning the stop cock, the mercury will rise to an equilibrium in both 
arms. 

231. How high will a spring rise? 232. How are fountains formed? How 
high will the water spout through the ducts ? What prevents the fluids from ris- 
ing to the same height with the reservoir ? 233. What is the syphon ? In what 
manner is the syphon used? How can the syphon be used to show the equilibri- 
um of fluids? How high will the liquid rise in each side of the syphon? What 
experiment, made with the syphon, is mentioned in the note. 



HYDRAULICS. 



77 




234. Tantalus' cup consists of a goblet containing a 
small figure of a man. A syphon is concealed within 
Fig. 60. the figure, which empties the water from the 
goblet as fast as it is poured in, so that the 
glass can never be filled. Fig. 60 represents 
the cup with the syphon. The figure of the 
man is not represented, in order that the po- 
sition of the syphon may be seen. 

235. Water, by means of its weight or its 
force when in motion, becomes a mechanical 
agent of great power. It is used to propel or turn 
wheels of different construction, which being connect- 
ed with machinery of various kinds, form mills, &c. 

236. There are three kinds of water-wheels, called 
undershot, overshot, and breast wheels. 

237. The Overshot wheel is awheel set in motion by 
the weight of water flowing upon it. It receives its 
motion at the top. 

Illustration. Fig. 61 represents the 
overshot wheel. It consists of a 
wheel turning on an axis, (not rep- 
resented in the Fig.) with compart- 
ments called buckets, abed, &c. at 
the circumference, which are succes- 
sively filled with water from the 
stream S. The weight of the water 
in the buckets causes the wheel to 
turn, and the buckets being gradually 
inverted are emptied as they descend. 
It will be seen," from an inspection of 
the figure, that the buckets in the de- 
scending side of the wheel are always filled, or partly filled, while 
those in the opposite or ascending part are alwavs emptv until they 
are again presented to the stream. This kind of wheel is the most 
powerful of all the water-wheels. 

238. The Undershot wheel is a wheel which is set in 
motion by the motion of the water. It receives its im 
pulse at the bottom. 




234. What is Tantalus' cup? What does Fi<*. 60 represent ? 235. How, and 
for what purposes is water used as a mechanical asent? 236. How many kinds of 
water-wheels are there ? What are they? 237 What is the overshot wheel? 
W here does it receive its motion? Explain Fi?. 61 What causes the wheel to 
turn . How does this wheel compare in power with the other water-wheels ? 238 
U hat is the undershot wheel ? W"here does it receive its motion ? 



s 



"8 



NATURAL PHILOSOPHY. 



Fig. 62. 




Illustration. Fig. 62 
represents the undershot 
wheel. Instead of buck- 
ets at the circumference, 
it is famished with plane 
surfaces, called float- 
boards, abed, &c. which 
receive the impulse of 
the water, and cause the 
wheel to revolve. 

239. The Breast 
wheel is a wheel 
which receives the 
water at about half its own height, or at the level of 
its axis. It is set in motion both by the weight and the 
motion of the water. 

Illustration. Fig. 
63 represents a breast 
wheel. It is furnish- 
ed either with buck- 
ets, or with float- 
boards, fitting the 
water course. 

In all the wheels 
, which have been de- 
scribed, the motion 
given to the wheel, 
is communicated to 
other machinery or , . . ttn „u A t n ihp 

-earing, as it is called, by other wheels or pinions attached to the 
axis, such as have been described in page 53, No. I u> 




SECTION XI. 



Pneumatics. 

240. Pneumatics treats of the nature mechanical 
properties, and effects of air and similar fluids, which 
are distinguished by the name of uniform fluids. 



What does Fig. 61 represent? How fa. Uhis wheel ^jftjyjftf JSK 

described, communicated* 240. Of what does Pneumatics treat . 



PNEUMATICS. 79 

241. The air which we breathe is an elastic fluid 
which surrounds the earth, and extends forty-five miles 
above its surface. It possesses many of the properties 
belonging to liquids in general, besides several others, 
the result, or, perhaps, the cause of its elasticity. Its 
specific gravity is eight hundred times less than that of 
water.* 

242. Air, steam, vapor, gas, are all elastic fluids 
possessing the same mechanical properties.! Whatev- 
er, therefore, is stated in relation to air, belongs in 
common to all of these fluids. 

243. Air and other similar fluids have weight, but 
their particles do not, under any circumstances, adhere 
together ; or, in other words, they are influenced by 
gravity, but have no cohesive attraction. J 

244. Air has two principal properties, namely, grav- 
ity, or weight, and elasticity. 

245. By the elasticity of the air is meant its power 
of increasing or diminishing in bulk, or extension, ac- 
cording as it is more or less compressed. § It is this 
property which distinguishes the aeriform fluids from 
liquids. 

* The air is necessary to animal and vegetable life, 2nd to combustion. It is 
a very heterogeneous mixture, being filled with vapors of an kinds. It CCHSIats, 
however, of two principal ingredients called oxygen and nitrogen, or azote ; of 
the former of which there are 2-3 parts, and of the latter, 72 in a hundred. The 
air is not visible, because it is perfectly transparent. It may be felt when it 
moves in the form of wind, or by swinging the hand rapidly backward and for- 
ward. 

t The chemical properties of liquids, fluids, &c. are not treated in the sciences 
of Pneumatics, Hydraulics, or Hydrostatics, but belong peculiarly to the science 
of chemistry. They are not, therefore, described in this work. 

X It has already been stated [See „Vb. 61.] that heat insinuates itself between 
the particles of bodies, and forces them asunder, in opposition to the attraction of 
cohesion and of gravity; it, therefore, exerts its power against both the attrac- 
tion of gravitation and the attraction of cohesion. But as the attraction of co- 
hesion does not exist in fluids in the form of air, (or aeriform fluids) the expan- 
sive power cf heat has nothing to contend wich but gravity. Any increase of 
temperature, therefore, expands an elastic fluid prodigiously, and a diminution of 
heat condenses it. 

§ The terms " rarefaction,'" and "rarified " are applied to air when it is expand- 
ed ; and " condensation ," or "condensed"' when it is compressed. It has already 

241. What is the sir which we breathe? How far dues it extend above 
the surface of the earth ? I)> es it possess properties common to liquids in 
general? How doe3 its specific gravity compare with that of water? Of 
what two principal ingredients does the air consist ? What is the proportion 
of these parts to each other ? 242. What other fluids are named belonging to 
the class of elastic fluids? 243. Have the air. and other similar fluids, weight .' 
With what power alone has heat to contend in aeriform fluids ? 244. What two 
principal properties has the air? 245. What is meant by the elasticity of -the 
air ? How do the aeriform fluids differ from liquids ? When is the air said to be 
rarified ! When condensed ! 



80 



NATURAL PHILOSOPHY. 



are double pumps,) and others only one 

Fig. 64. 



246. The air pump is an instrument by means of 
which the air may be pumped or drawn from a vessel 
prepared for the purpose. The vessel is called a re- 
ceiver, and is made of glass, in order that the effects 
of the removal of the air may be seen. 

Illustration. Air pumps are made in various ways, and are of 
different constructions. Some have two barrels, (or, in other words, 
a™ rlnnhlp nnnrmc n *nri ^tv^orc «„i„ ™~ The difference between 

them will be present- 
ly explained. Fig. 64 
represents a single 
barrel air pump,* used 
both for condensing 
and exhausting. A D 
is the stand or plat- 
form of the instru- 
ment, which is screw- 
ed down to the table 
by means of a clamp, 
underneath, which is 
not represented in the 
figure. R is the glass 
vessel or bulbed receiv- 
er from which the air 
is to be exhausted. P 
is a solid piston, accu- 
rately fitted to the but e 
of the cylinder, and H 
the handle by which it is moved. The dotted line, F, represents the 
communication between the receiver R and the barrel B ; it is a 
tube through which the air, entering at the opening 1, on the plate 
of the pump, passes into the barrel, through the exhausting valve 
e v. c v is the condensing valve, communicating with the barrel B 
by means of an aperture near e, and opening outwards through 
the condensing pipe P. 

been stated [See page 22, JVb.82] that the air near the surface of the earth bears the 
weight of that which is above it. Being compressed, therefore, hy the weight of 
that above it, it must exist in a condensed form near the surface of the earth, while 
in the upper regions of the atmosphere, where there is no pressure, it is highly rari- 
fied. This condensation, or pressure, is very similar to that of water at great 
depths in the sea. [See No. 207.] 

* The air pump, described in this figure, is one of a number made by A. & D. 
Davis, of this city, by order of a special committee, for the Boston Schools. It 
has a piston of large size, being an inch and a half in calibre. The pneumatic 
instruments, mentioned in this section, belong to the same set, and are from the 
same manufacturer. There are several other manufactories of philosophical in- 
struments, in the city, which deserve commendation, among which may be men- 
tioned those of Mr. T. Claxton, and Mr. Chamberlin. 



Is the air, near the surface of the earth, rare or dense? 246. What is the 
use of the air pump? What fig. represents an air pump ? Explain the figure. 







PNEUMATICS. 81 

The operation of the pump is as follows: The piston P being 
drawn upwards by the handle H, the air in the receiver R, by its 
elasticity expanding, passes by the aperture I through the tube T, 
and through the exhausting valve e v into the barrel. On the de- 
scent of the piston the air cannot return through that valve, because 
the valve opens upwards only; it must, therefore, pass through the 
aperture by the side of the valve and through the condensing valve 
c v into the pipe P, where it passes out into the open air. It cannot 
return through the condensing valve c v, because that valve opens 
outwards only. By continuing this operation, every ascent and de- 
scent of the piston P must render the air within the receiver R more 
and more rare, until its elastic power is entirely exhausted. The 
receiver is then said to be exhausted ; and although it still contains 
a small quantity of air, yet it is in so rare a state that the space 
within the receiver is considered a. vacuum* 

From the explanation which has been given of the operation of 
this air pump, it will readily be seen that by removing the receiver 
R and screwing any vessel to the pipe P, the air willjbe condensed 
in the vessel. Thus the pump is made to exhaust or to condense, 
without alteration.t 

The double air pump differs from the single air pump in having 
two barrels and two pistons ; which instead of being moved by the 
hand, are worked by means of a toothed wheel, playing in notch- 
es of the piston rods. 

247. By means of the air pump the following facts 
are illustrated. First, That the air has weight. Sec- 
ondly, that it is susceptible of almost unlimited expan- 
sion. Thirdly, that it can also be condensed, or crowd- 
ed into much smaller dimensions than it naturally has.f 

* Properly speaking, a vacuum is a space entirely empty, having neither air nor 
any other substance in it. From the explanation now given of the operation of 
the air pump, it will be seen, that that instrument is incapable of producing a 
perfect vacuum. But the air within the receiver is so exceedingly rare, when 
thus exhausted, that, for all practical purposes, it may be considered a vacuum. 
The only mode of producing a perfect vacuum is by means of the Torricellian 
experiment, on the principle of the Barometer, which will be explained hereafter. 

■f The piston and valves of the air pump should be kept well oiled. All the 
brass work, in the Boston School set, being lackered need not be polished : but all 
those parts which come into contact with water should be wiped dry after they 
have been used. 

\ This property is not illustrated by common air pumps, but is exhibited by an 
instrument called a condensing syringe, or condenser. The peculiar construction of 
the air pump, belonging to the " Boston School set " of philosophical instruments, 
as has already been shown, adapts the instrument both for exhausting at:d con- 
densing, and thereby supplies the place of a separate instrument for condensing. 
The condensing syringe is, in fact, nothing more than the air pump reversed, by 
which air is diiven into any vessel instead of being drawn out The valve, 
therefore, opens inwards in respect to the vessel, instead of outwards, as the ex- 
hausting pump is constructed. 

Explain the operation of the pump. What is meant by a vacuum ? In what 
way can a perfect vacuum be produced ! How does the double air pump differ 
from the single.' What facts are illustrated by means of the air pump? 

8* 



82 



NATURAL PHILOSOPHY. 



Fig. 65. 




Experiments to be made with the air pump. 1. Place the glass re- 
ceiver R, as represented in fig. 64, upon the pump plate, and exhaust 
the air from under it, by working the piston up and down. The 
receiver will adhere strongly to the plate. But if the air be re-ad- 
mitted by turning the screw S, the receiver may easily be raised. 
This experiment shows the pressure of the atmosphere, caused by 
its weight. 

2. Fig. 65 represents the hcnd glass. It is, in 
fact, nothing more than a tumbler, open at both 
ends, with the top and bottom ground smooth, so 
as to fit the brass plate of the air pump. Put it 
upon the plate, and cover it closely with the palm 
of the hand, and work the pump. " The air with- 
in the glass being thus exhausted, the hand will 
be pressed down by the weight of the air above 
it; and the pressure felt upon that portion of the 
hand over the glass will be equal to 14 or 15 
pounds to every square inch. This experiment, 
likewise, shows the pressure of the air. 
3. Place a small bladder, partly filled with air, and lightly closed, 
under the glass receiver, and, on working the pump, thus removing 
the air from around the bladder, the air within will gradually ex- 
pand, and cause the bladder to appear full. On turning the screw 
S and re-admitting the air, the bladder will immediately resume its 
shrivelled appearance. The same effect may be produced on a dried 
apple, or raisin, if the skin be whole. This experiment shows the 
elasticity of the air. 

4. Fig. 66 represents a stop cock, r of 
which there are two, of different sizes, 
with a screw fitted to the aperture I in 
the brass plate, or to the pipe near the 
condensing valve c v in front of the 
pump. By inserting the stop cock in- 
to an india rubber bag, or fitting a 
bladder to it, and screwing it into the pipe in front, and working the 
pump, air will be condensed into it. When this is done, remove 
the bag or bladder to the screw in the brass plate, and place another 
bag on the condensing pipe. On working the instrument, the air 
will be conveyed from the full to the empty bag or bladder. Thus 
the pump is made to exhaust and condense at the same time. 

5. Fig. 67 represents the elastic tube. Screw the elastic tube in- 
Fig. 67. to the pump plate, and connect the oth- 

er end by the stop cock, with the glass 
syphon, [See Fig. 59.] immersed in 
mercury. On working the pump the 
mercury will rise in the syphon to the 

1. What is the first experiment mentioned, to be made with the air pump! 
What fig. represents it? What does this experiment show ? 2. What is the sec- 
ond experiment ! What fig. lepresents the hand glass ? To what is the pressure, 
felt upon that portion of the hand over the glass, equal ? What does this experi- 
ment show? 3. What is the third experiment mentioned? What does this ex- 
periment show: 4. What does fig. 66 represent? What is the fourth experi- 
ment? What may be shown from this > 5. What does fig. 67 represent ? What 
is the fifth experiment ? 




PNEUMATICS. 06 

height of more than twenty-eight inches, showing that the upward 
pressure of the atmosphere is equal to this height of mercury.* 

6. With the elastic tube still attached to the air pump, and the 
svphon, as in the last experiment, the stop cock being open ; stop the 
other end of the syphon with the finger— exhaust the air— then 
close the stop cock— now insert the end of the syphon, which is 
stopped with the finger, into a bowl of water, and remove the finger 
—the water will immediately fill the whole length of the syphon, 
and would rise thirty-three feet, were the syphon as long. 

Fig 68# 7. Fig. 68 represents the instrument 

forraisng a weight by the upward pres- 
sure of the air. It consists of a glass 
tube, of large bore, set in a strong case 
or stand, supported by three legs. A pis- 
ton is accurately fitted to the bore of the 
tube, and a hook is attached to the bot- 
tom of the piston from which weights 
are to be suspended. One end of the 
elastic tube is to be screwed to the plate 
of the pump, and the other end attached 
to the top of this instrument. The air 
being ihen exhausted from the tube, the 
weights will be raised the whole length 
of the glass. The number of pounds 
weight that can be raised by this instru- 
ment maybe estimated by multiplying the number of square inches 
in the bottom of the piston, by fifteen. This experiment shows the 
upward pressure of the air. 

8. Fig. 69 is a bell-shaped glass, covered with a piece of bladder, 
which is tied tightly around its neck. Thus prepared it may be 
screwed to the plate of the air pump, or connected with it by means 
of the elastic tube. On exhausting the air from the glass, the exter- 
nal pressure of the air on the bladder will 
burst it inwards with a loud explosion. The 
experiment may be reversed, and the bladder 
burst, by condensing air within the glass. For 
this purpose, transfer the glass or the elastic 
tube, connected with it, to the condensing tube 
P, and, on working the pump, the air will be 
condensed within the glass, and by its pressure 
burst the bladder outwards, with a loud explo- 

* This experiment furnishes a test of the power of the pump. Caution is ne- 
cessary in disengaging the syphon from the flexible tube, or taking it out of the 
mercury. In all cases the thumb screw of the air pump should be turned, and air 
admitted before removing the syphon, &c, otherwise, the air, rushing in at the 
lower end of the syphon, will force the mercury violently into the air pump, and 
probably break the syphon. 





What does this experiment show ? 6. What is the sixth experiment >. 7. What 
does fig. 68 represent >. Of what does it consist ! How is it used ? How can the 
number of pounds weight, that can be raised by th'i3 instrument, be estimated? 
What does this experiment show ? 8. What does fig. 69 represent? What ex- 
periments are mentioned in No. 8 ? 



84 



NATURAL PHILOSOPHY. 



Fig. 70. 




sion. The former experiment is the result of the gravity— the lat- 
ter of the elasticity of the air. 
9. Fig. 70 is a glass similar to the one represented in the last 
figure, covered with india rubber. The same 
experiments may be made with this as were 
mentioned in the last article, but with differ- 
erent results. Instead of bursting, the india 
rubber will be pressed inwards the whole 
depth of the glass, when the air is exhausted; 
and will swell outwards like an inflated blad- 
der when the air is condensed in the glass. 

10. Fig. 71 is called the guinea and feather 
drop. In most collections of philosophical ap- 
paratus this instrument consists of a tall receiver with brass shelves 
near the top, on which a guinea and a feather may be placed. The 
Fig. 71. air being exhausted, and a screw on the top being turn- 
ed, the shelves drop and cause the guinea and feather 
to fall together. This instrument is designed to show 
how falling bodies are retarded by the resistance of 
the air. When the air is within the receiver, the guin- 
ea will fall first, while the feather, being retarded 
by the resistance of the air, falls slowly ; but when the 
air is exhausted they will both reach the bottom at the 
same moment. The instrument represented in the fig- 
ure is the one belonging to " the Boston School set" and 
is of different construction. It consists of a large glass 
TS** tube, sealed at one end, and fitted for the reception of 
^y the stop cock (See Jig. 66.) at the other. A feather and 

a small piece of brass (in lieu of the guinea,) are en- 
closed in it. Before exhausting the air it should be turned several 
times to show that the heavy body (namely, the brass) will fall first. 
It should tl\en be screwed to the plate of the pump, the air exhaust- 
ed, and the stop cock closed. On removing it from the pump and 
turning it up, it will be seen that both the feather and the brass will 
fall together, and reach the bottom at the same time. 
Fig. 72. 11. Fig. 72 represents the straight jet, 

which is a small brass tube. Fig. 73 is the 
fountain glass. The experiment with these 
instruments is designed to show the pres- 
sure of the atmosphere on the surface of 
liquids. Screw the stop cock to the plate 
of the air pump, then screw the straight jet 
into the stop cock, and the fountain glass 
over the jet to the cock. Having exhausted 
*lf the air from the fountain glass, turn the 
^ stop cock, and removing it with the glass 
from the pump, and immersing it in a ves- 

Of what is the first experiment the result? The second ? 9. What does fig. 
70 represent >. How do the experiments, mentioned in No. 9, differ from those of 
Vo. 8. 10. What does fi<r. 71 represent ? What is this instrument designed to 
show! When will the guinea fall first ! When will ihey both fall at the same 
ime> 11. What do figs. 72 and 73 represent? What are the experiments, 
nade by this instrument, designed to show .' What experiment is mentioned 



Fig. 



PNEUMATICS. 



85 



sel of water, open the stop cock. The pressure of the air on the 
surface of the water will cause it to rush up into the glass like a 
fountain. 



12. Fig. 74 represents the flask or glass 
vessel and scales for weighing air. Screw 
the stop cock to the flask, and, hanging it 
to the hook under the shorter scale, ascer- 
tain the weight of the flask while it is open, 
and, of course, filled with air — then, hav- 
ing screwed it into the pump plate, and 
exhausted the air, again weigh the flask. 
The difference between its present and 
former weight is the weight of the air that 
was contained in the flask. * 



Fig. 74. 




Fig. 75. 




13. Fig. 75 is a hollow, brass 
globe, or condensing chamber, 
for condensing air. Having part- 
ly filled it with water and insert- 
ed the stop cock, screw it to P, 
the condensing pipe of the pump, 
and condense the air ; then turn 
the stop cock to confine the air, 
and, removing the globe from the 
pump, insert the straight jet (Fig. 
72) into the stop cock; and, on 

turning tbp rock, the pressure of 

the air within the globe will force 
the water out in a beautiful 
stream, and with great force. 
Fis. 



The same experiment may be 
performed with the revolving jet represented in fig. 76. The water 
will form a beautiful circle in the air as it is forcibly ejected from 
the jet, and the tube will rapidly revolve. 

* CondeDsed air may, likewise, be weighed in the brass globe, after being intro- 
duced as described in the next experiment. In weighing air, the temperature of 
the room must be observed, because heat rarifios it and renders it lighter ; there- 
fore the warmer the room in which the experiment is tiied, the lighter the air 
■will be. 



12. What does fig. 74 represent : What experiment is mentioned? How can 
condensed air be weighed ; What, is necessary to he observed in weighing air I 
13. What does fig. 75 represent? What experiment is meutioned .' What does 
fig. 76 represent? 



86 



NATURAL PHILOSOPHY. 



14. Fig. 77 is a small brass cy - 
inder or gun barrel, with a screw 
fitted to the brass globe, designed 
to show the operation of an air 
gun.* Screw the brass globe, with 
the stop cock, to the condensing 
pipe of the air pump and condense the air. Turn the stop cock, 
remove the globe from the condensing pipe, and screw the gun bar- 
rel to the stop cock — put a lead shot, or paper ball, into the barrel, 
and quickly open the stop cock — the shot will be thrown with force 
across the room. 

Fig. 78. 

15. Fig. 78 represents the straight receiver.t Fill the 
straight receiver with water, and placing it on the plate 
of the air pump, cover it with the bulbed receiver, and 
exhaust the air. The air contained within the water 
will then rise in bubbles, and, escaping from the surface, 
present the appearance of boiling water. 

16. "With the two receivers, as in the last experiment, 
sink a piece of wood in the water, and, on exhausting 
the air from the water, the air will be seen issuing from 
the pores of the wood. 

17. Fig. 79 represents the glass balloon. With the re- 
ceiver, as in the last two experiments, place the balloon 
with the neck downwards upon the surface of the wa- 
ter. (It will, perhaps, be necessary to admit a little 
"water into the balloon to make it stand in the water.) 

On eAliaustiiigthe air, the air will be seen iocuiug fiom 

the balloon. The air being admitted into the receiver, 
the balloon will sink, or, again exhausting the air the 
balloon will rise. This experiment may be repeated at 
pleasure. 

18. Ether, alcohol, and other distilled liquors, or boil- 
ing water, placed under the receiver, will appear to boil 
when the air is exhausted. 

19. Place a lighted taper, cigar, or any other sub- 
stance, that will produce smoke, under the receiver, and 
exhaust the air. — the light will be extinguished and 
the smoke will fall, instead of rising. If the air be re- 
admitted the smoke will ascend. 





* Condensed air, by its elastic force, will produce effects similar to gun-powder. 
Air guns have been constructed from which shot, may he thrown with a force al- 
most as great as that of gunpowder With the air gun, a bullet may he made to 
perforate a board. With the brass globe and the cylinder or gunnel barrel, de- 
scribed in fig. 77, the operation of the air gun may easily be understood. 

| This vessel properly belongs to the electrical apparatus, and for this reason it 
is coated with strips of tin foil, like a Leyden jar. It is used with the pneumatic 



14. What does fig. 77 represent? What is its design; What is said, with re- 
gard to condensed air, in the note ? What experiment is mentioned ? 15. What 
loes fig. 78 represent ? What experiment is here mentioned? 16. What experi- 
nent is mentioned in No. 16. 17. What does fig 78 represent? What experi- 
nent is here mentioned ? 18. What is stated in Nos. 18 and 19: 




PNEUMATICS. 87 

20. Fill the straight receiver with water, cover it closely with 
paper, and invert it— the paper will be held on by the upward pres- 
sure of the air, although it sustains the whole weight of the water. 
Fig. 80. 21. Fig. 80 represents the Magdeburgh cups 

<^-^T\ or hemispheres. They consist of two hol- 

vv^___^/ low brass cups, the edges of which are accu- 

rately fitted together. They each have a han- 
dle, to one of which the stop cock is fitted. 
The stop cock, being attached to one of the 
cups, is to be screwed to the plate of the air 
pump, and left open. Having joined the oth- 
er cup to that on the pump, exhaust the air 
from within them, turn the stop cock to pre- 
vent its readmission, and screw the handle 
that had been removed to the stop cock. 
Two persons may then attempt to draw these 
cups asunder. It will be found that great 
power is required to separate them ; but, on 
readmitting the air between them, by turn- 
ing the cock, they will fall asunder by their 
own weight. When the air is exhausted from 
within them, the pressure of the surrounding air upon the outside 
keeps them united. This pressure, as has already been stated, is 
equal to a pressure of fifteen pounds on every square inch of the 
surface. Whence it follows that the larger the cups or hemispheres 
the more difficult it will be to separate them. 

22. By means of a weight, sink a small bladder, partly filled with 
air, and tightly closed, in water contained in the straight receiver — 
cover it with the bulbed receiver, and exhaust the air — as the sur- 
rounding pressure is thus removed the air within the bladder will 
expand, and its specific gravity being thus diminished,, it will rise. 
On re-admitting the air it will sink again. 

23. If an animal be placed under the receiver, and the air ex- 
hausted, it will immediately droop, and if the air be not speedily re- 
admitted it will die. 

24. A simple and interesting experiment, connected with the sci- 
ence of chemistry, may thus be performed by means of the air 
pump. A watch glass, containing water, is placed over a small- 
vessel containing sulphuric acid, and put under the bulbed receiver- 
When the air is exhausted, vapor will freely rise from the water r 
and be quickly absorbed by the acid. An intense degree of cold is 
thus produced, and the water will freeze. 

In the above experiment if ether be used instead of the acidL 

instruments in " the Boston School set," under the name of the straight receiver* 
on account of its small size, which allows the bulbed receiver to cover it. Econ- 
omy suggests the application of each instrument to as many purposes as it car* 
conveniently answer. 

20. What is stated in No. 20? 21. What does fig. 80 represent ? What exper- 
iment is here mentioned ! Does the size of the cups have any effect upon their 
separation? 22. What experiment is here mentioned .' 23. What one is men- 
tioned in this number? 94. What experiment, connected with the scieace of 
chemistry, is mentioned ? 



88 NATURAL PHILOSOPHY. 

the ether will evaporate instead of the water, and in the process of 
evaporation, depriving the water of its heat, the water will freeze. 
These two experiments, apparently similar in effects, namely, the 
freezing of the water, depend upon two different principles which 
pertain to the science of chemistry. 

The following experiments may be made with the syphon. [See Fig. 
59.1 

25. Screw the stop cock into the shorter end of the syphon, close 
the stop cock, and pour mercury or water into the longer side. The 
air contained in the shorter side will prevent the liquids from ris- 
ing in the shorter side. But if the stop cock be opened so as to af- 
ford free passage outwards for the air, the fluids will rise to an equi- 
librium, that is, to equal height in both arms of the syphon. 

26. With water or mercury in the syphon, and the stop cock 
•open, turn the syphon so that the fluid will enter the shorter arm, 
and when that arm is filled up to the stop cock, close the stop cock to 
prevent the admission of the air : the syphon may then be turned 
in any direction and the fluid will not run out, on account of the 
pressure of the atmosphere against it. But if the stop cock be open- 
ed, the fluid will run out freely. 

27. With a quantity of water in the balloon, (Fig. 79.) or a weight 
attached to it sufficient to render its specific gravity nearly the same 
with that of water, immerse it in a tall vessel full of water, and let 
it float on the surface. Cover the top of the vessel closely with In- 
diarubber, or any elastic covering. On pressing the covering with 
the hand, the balloon will immediately descend in the water, and 
when the pressure is removed it will again float about, rising or fall- 
ing, oi standing still, according to the pressure on the covering. 

This experiment may be thus explained : — the pressure on the 
top of the vessel first condenses the air between the cover and the 
surface of the water — this condensation presses upon the water be- 
low, and as this pressure affects every portion of the water through- 
out its whole extent, the water, by its upward pressure, compresses 
the air within the balloon, and makes room for the ascent of more 
water into"the balloon so as to alter the specific gravity of the balloon, 
and cause it to sink. As soon as the pressure ceases, the elasticity of 
the air in the balloon drives out the lately entered water, and re- 
storing the former lightness to the balloon causes it to rise. If, in 
the commencement of this experiment, the balloon be made to have 
a specific gravity too near that of water, it will not rise of itself, 
after once reaching the bottom, because the pressure of the water 
then above it will perpetuate the condensation of the air which 
caused it to descend. It may even then, however, be made to rise, if 
the parpendicular height of the water above it be diminished by in- 
clining the vessel to one side. 

What will be the cffcct if ether be used instead of the acid? 25. What exper- 
iment, marie with the syphon, is mentioned in No. 25 ? 26? 27? How would 
you explain this experiment? What will be the effect if the balloon, in the com- 
mencement of this experiment, bo mala to hive a specific gravity too near that of 
water » Why will it not rise I Kow can it be made to rise.' What does this 
experiment prove > 



PNEUMATICS. »y 

This experiment proves many things ; namely: 

First. The materiality of air, by the pressure of the hand on 
the top being communicated to the water below through the air in 
the upper part of the vessel. 

Secondly. The compressibility of air, by what happens in the 
globe before it descends. 

Thirdly. The elasticity, or elastic force of air, when the water is 
expelled from the globe, on removing the pressure. 

Fourthly. The lightness of air, in the buoyancy of the globe. 

Fifthly. It shows that the pressure of a liquid is exerted in all di- 
rections, because the effects happen in whatever position the jar be 
held. 

Sixthly. It shows that pressure is as the depth, because less pressure 
of the hand is required, the farther the globe has descended in the 
water. 

Seventhly. It exemplifies many circumstances of fluid support. A 
person, therefore, who is familiar with this experiment,* and can 
explain it, has learned the principal truths of hydrostatics and 
pneumatics. 

248. Air may become a mechanical agent by means 
of its four properties, weight, inertia, fluidity, (or pow- 
er of transmitting pressure,) and its elasticity. 

249. The pressure of the air, (as has already been 
stated) caused by its gravity or weight, is equal to fif- 
teen pounds on every square inch of any surface ; hence 
it is calculated that a man of common stature has to 
sustain a weight of about fourteen tons of air. The 
equality of the pressure on every part of his body pre- 
vents his being injured by this immense weight; and 
the air contained within the body and its pores, also 
counterbalances the weight of the external air. If this 

* On the same principle with the balloon, described in this experiment, several 
images of glass, hollow within, and each having a small opening at the heel by 
which water may pass in and out, may be made to manoeuvre in a vessel of water. 
Place them in a vessel in the same manner with the balloon, but by allowing dif- 
ferent quantities of water to enter the apertures in the images, cause them to dif- 
fer a little from one another in specific gravity. Then, when a pressure is exerted 
on the cover, the heaviest will descend first, and the others follow in the order of 
their specific gravity ; and they will stop or return to the surface in reverse order, 
when the pressure ceases. A person exhibiting these figures to spectators, who 
do not understand them, while appearing carelessly to rest his hand on the cover 
of the vessel, seems to have the power of ordering their movements by his will. 
If the vessel, containing the figures, be inverted, and the cover be placed over a 
hole in the table, through which, unobserved, pressure can be made by a rod rising 
through the hole, and obeying the foot of the exhibiter, the most surprising evolu- 
tions may be produced among the figures, in perfect obedience to the word of com- 
mand. 

First? Secondly? Thirdly.' Fourthly? Fifthly? Sixthly? Seventhly J What ex- 
periment is mentioned in the note .' 248. How may air become a mechanical agent? 
249. To what is the pressure of the air, on every square inch, equal '. What 
weight of air does a common sized man sustain? "Why does not this weight in- 
jure him I 

9 



90 NATURAL PHILOSOPHY. 

external pressure were removed, the air within the 
body, meeting with no external pressure to restrain its 
elasticity, would burst the parts which confine it, and 
destroy life. This pressure is proved by experiments 
numbers 2, 8, and 21, pages 82, 83, and 87. 

250. A vacuum is a space from which the air and 
every other substance has been removed. 

251. The resistance which light bodies meet from 
the air, causes them to fall slowly, while heavy bodies, 
more readily overcoming this resistance, fall rapidly. 
This is proved by experiment number 10, page 84. 

252. When the external pressure is removed from 
any portion of confined air, it will immediately expand, 
and to this expansion there are no known limits. See 
experiment number 3, page 82. 

253. A column of air reaching to the top of the at- 
mosphere, the base of which is a square inch, weighs 
fifteen pounds, when the air is heaviest. [See No. 246.) 
Therefore, as all fluids press equally, in all directions, 
every inch of our bodies sustains a weight of fifteen 
pounds. The exact pressure that any individual sus- 
tains may, therefore, be ascertained by finding the 
number of square inches there are on the surface of 
his body, and multiplying it by fifteen. In like man- 
ner, the weight of the whole atmosphere* may be 
ascertained by finding the number of square inches 
there are on the surface of the globe, and multiplying 
the same by fifteen. In this way it has been ascertain- 
ed that the weight of the whole atmosphere is more 
than five thousand billions of tons. 

* The exact height, to which the atmosphere extends, has never been accurately 
ascertained ; but it ceases to reflect the sun's rays at a greater height than forty- 
five miles. It has been computed that the weight of the whole atmosphere is 
equal to that of a globe of lead, sixty miles in diameter. 

What would be the effect if this external pressure were removed ? 250. What is 
a vacuum ? 251. Why do light bodies fall more slowly than heavy > How is this 
proved? 252. What will be the effect if ihe external pressure be removed from 
any portion of confined air >. Are there any known limits to this extension .' 253. 
What is the weight of a column of ait, the base of which is a square inch, reach- 
ing to the top of the atmosphere, when the air is heaviest ? What weight does 
every inch of our bodies sustain ? How can the weight, of the whole atmosphere 
be ascertained ? Has the exact height to which the atmosphere extends been ac- 
curately ascertained ? At what height does it cease to reflect the sun's rays. To 
what is the computed weight of the atmosphere equal ? 



PNEUMATICS. 



91 



254. The air consists of particles possessing the in- 
herent properties of matter. It, therefore, has the 
property of impenetrability, (See No's. 23 and 24.) like 
all other substances. 

Illustration. If a tube, closed at one end, or an inverted tumbler, 
be inserted at its open end, in a vessel of water, tbe water will not 
rise in the tube or tumbler, to a leve* with the water in the vessel, 
on account of the impenetrability of the air within the tube. But 
if the tube be open at both ends, the water will rise, because the air 
can escape through the upper end. It is on this principle that the 
diving bell (or the diver's bell, as it is sometimes called,) is con- 
structed. 

Fig- 81. Fig. 81 represents a diving bell. It con- 

sists of a large heavy vessel, formed like a 
bell, (but may be made of any other shape,) 
with the mouth open. It descends into the 
water with its mouth downwards. The air 
within it having no outlet is compelled, 
by the order of specific gravities, to ascend 
in the bell, and thus (as water and air can- 
not occupy the same space at the same time,) 
prevents the water from rising in the bell 
A person, therefore, may descend with safe- 
ty in the bell to a great depth in the sea, and 
thus recover valuable articles that have been 
lost. A constant supply of fresh air is sent 
down, either by means of barrels, or by a 
forcing pump similar to the condenser of the 
air pump. In the fig., B represents^the bell 
with the diver in it. C is a bent metallic 
tube attached to one side and reaching the 
air within ; and P is the forcing pump 
through which air is forced into the bell. 
The forcing pump is attached to the tube by 
a joint at D. 

When the bell descends to a great depth, 
the pressure of the water condenses the air 
within the bell, and causes the water to as- 
cend in the bell. This is forced out by con- 
stant accessions of fresh air, supplied as above mentioned. Great 
care must be taken that a constant supply of fresh air is sent down, 
otherwise the lives of those within the" bell will be endangered. 
The heated and impure air is allowed to escape through a stop 
cock in the upper part of the bell. 




254. Of what does the air consist ? What follows from this ? What illustra- 
tion of this is given ! Upon what principle is the diving hell constructed .' What 
fig. represents the diving bell? Why does not the water rise in the bell; Ex- 
plain the figura. 



92 NATURAL PHILOSOPHY. 

255. The Barometer, or weather glass,* is an in- 
strument to measure the weight or pressure of the at- 
mosphere, and foretel the variations of the weather. 

256. The Thermometert is an instrument to measure 
the heat of the air. 

257. The Hygrometer f is an instrument to measure 
the degree of moisture in the air. 

Fig. 82 represents a barometer. It consists of a 
Fig. 82. long glass tube, about thirty-three inches in length, 
closed at the upper end and filled with mercury. 
n The tube is then inverted in a cup, or leather bag, of 

I jf3'0 mercury, on which the pressure of the atmosphere is 

exerted. As the tube is closed at the top it is evi- 
dent that the mercury cannot descend in the tube 
without producing a vacuum. § The pressure of the 
atmosphere (which is capable of supporting a col- 
umn of mercury of about 28 or 30 inches in height) 
prevents the descent of the mercury ; and the instru- 
ment, thus constructed, becomes an implement for 
ascertaining the weight of the atmosphere. As the 
air varies in weight or pressure, it must, of course, 
influence the mercury in the tube, which will rise or 
fall in exact proportion with the pressure. When 
MMf the air is thin and light, the pressure is less, and 

^P the mercury will descend ; and when the air is dense 

and heavy, the mercury will rise. At the side of the 
tube there is a scale, marked inches and tenths of an 
inch, to note the rise and fall of the mercury. 1 1 

* The word Barometer signifies the measure of weight. 

fThe word Thermometer means the measure of heat. 

% The word Hygrometer means the measure of moisture. 

§ The vacuum produced hy inverting a lube of mercury thus closed at the top, 
is called the Torricellian vacuum, from Torricelli, an Italian philosopher, who first 
discovered this means of producing it. This method produces the most perfect 
vacuum that can be formed. 

|| Any other fluid may be used as well as mercury, provided the length of the 
tube be extended in proportion to the specific gravity of the fluid. Thus, a tube 
filled with water must be 33 feet Ion;, because "the atmosphere will support a col- 
umn of water of that height. Mercury is used, therefore, in the construction of 
the barometer, because it does not require so long a tube as any other fluid. It 
may here be remarked that the air is the heaviest, and that, consequently, the 



255. What i? a barometer ? What dops the word barometer mean ? 256. What 
is a thermometer ? What does the word thermometer mean ! 257. What is a hy- 
grometer ! What does the word hygrometer mefin ! What figure represents a ba- 
rometer ! Explain its construction. What is said, in the note, with regard to the 
vacuum produced by inverting a tube of mercury thus closed at the top: What 
height, of mercury is the pressure of the atmosphere capable of sustaining; What 
effect has the pressure of the atmosphere on the mercury in the lube? In what 
proportion does the mercury rise and fall ! In what way can barometers be made 
of other fluids? Why is mercury used in preference to any other fluid? Is the. 
air heaviest, in wet or dry weather ? 



PNEUMATICS. 



93 



258. The pressure of the atmosphere on the mercu- 
ry, in the bag or cup of a barometer, being exerted on 
the principle of the equilibrium of fluids, (See num- 
ber 201,) it must vary according to the situation in 
which the barometer is placed. For this reason it 
will be the greatest in valleys and low situations, and 
least on the top of high mountains. Hence, the ba- 
rometer is often used to ascertain the height of moun- 
tains, and other places above the level of the sea.* 
Fi"-. 83. Fig. 83 represents a thermometer. (See No. 256.) 

In appearance it resembles a barometer, but it is con- 
structed on a different principle, and for a different 
purpose. It consists of a capillary tube, closed at the 
top, and terminated with a bulb, which is filled with 
mercury. T As heat expands and cold contracts most 
substances, it follows that in warm weather the mer- 
cury must be expanded and will rise in the tube, and 
that in cold weather it will contract and sink. Hence 
the instrument becomes a correct measure for the heat 
and cold of the air. A scale t is placed at the side of 
the tube, to mark the degree of heat or cold, as it is in- 
dicated by the rise and fall of the mercury in the ca- 
pillary tube. 

mercury will rise highest in dry weather. In wet weather the 
dampness renders the air less salubrious, and it appears, therefore, 
more heavy then, although it is, in fact, much lighter. 

* As the air diminishes in density, upwards, it follows that it 
must be more rare upon a hill than on a plain. In very eleva- 
ted situations it is so rare that it is scarcely fit for respira- 
tion, or breathing ; and the expansion which tukes place in the 
more dense air contained wilhin the body is often painful : it occa- 
sions distension, and sometimes causes the bursting of the smaller 
blood-vessels, in the nose and ears. Besides, in such siiuations, we 
are more exposed both to heat and cold ; for, though the atmos- 
phere is itself transparent, its lower regions abound with vapors 
and exhalations f rom the earth, wlvich float in it, and act, in some degree, as a 
covering, which preserves us equally from the intensity of the sun's rays, and 
from the severity of the cold. 

t Any other liquid which is expanded by heat and contracted by cold, such as 
spirits of wine, &c. will answer as well as mercury. 

J There are several different scales applied to the thermometer, of which those 
of Fahrenheit, Reaumur, Delisle and Celsius are the principal. The thermometer, 
in common use in this country, is graduated by Fahrenheit's scale, which, com- 
mencing with 0, or zero, extends upwards to 212°, the boiling point of water, and 
downwards to 20 or 30 degrees. The scales of Reaumur and Celsius fix zero at 
the freezing point of water ; and that of Delisle at the boiling point. 

258. On what principle is the pressure of the atmosphere on the mercury, in the 
cup of a barometer, er.erted > What follows from this ; For what other purpose, 
beside measuring the pressure of the atmosphere and foretelling the variations of the 
weather, is the barometer used > Is the air the more dense, at the surface of the 
earth or upon a hill ? What figure represents a thermometer ? Explain its con- 
struction. What effect have heat and cold on most substances ? What follows 
from this c Whose scale is generally used in this country.' 

9* 



94 NATURAL PHILOSOPHY. 

The hygrometer, for measuring the degree of moisture in- the 
air,* may be constructed of any thing which contracts and expands 
by the moisture and dryness of the atmosphere— such as most 
kinds of wood ; catgut, twisted cord, the beard of wild oats, &c. 

259. The pressure of the atmosphere on the surface 
of a well, or any other portion of water, is the means 
by which water is raised by the common pump. By the 
act of pumping, the pressure of the atmosphere is re- 
moved from the water within the body of the pump, and 
the water, consequently, will rise. 

. Fig. 84 represents the common pump, called the sucking pump. 
The body consists of a large tube,or pipe, the lower end of which 
is to be immersed in the water which it is designed to raise. P is 
the piston, v a valve in the piston, which, opening upwards, ad- 
mits the water to rise through it, but prevents its return. Y is a 
similar valve in the body of the pump, below the piston. When the 
pump is not in action the valves are closed by their own weight ; but 
when the piston is raised it draws up the column of water which rest- 
ed upon it, producing a vacuum between the piston and the lower 
valve Y. The water below, immediately rushes through the lower 
valve, and fills the vacuum. When the piston descends a second 
time, the water in the body of the pump passes through the valve v, 
and on the ascent of the piston is liftedtup by the piston, and a vacu- 

* By the action of the sun's heat upon the surface of the earth, whether land 
or water, immense quantities of vapor are raised into the atmosphere, supplying 
materials for all the water which is deposited again in the various forms of dew, 
fog, rain, snow and hail. Experiments have been made to show the quantity of 
moisture thus raised from the ground by the heat of the sun. Dr. Watson found 
that an acre of ground apparently dry, and burnt up by the sun, dispersed into 
the air sixteen hundred gallons of water in the space of twelve hours. His exper- 
iment was thus made: he put a glass, mouth downwards, on a grass plot, on 
which it had not rained for above a month. In less than two minutes the in?ide 
was covered with vapor ; and in half an hour drops began to trickle down its in- 
side. The mouth of the glass was twenty square inches. There are 121)6 square 
inches in a square yard, and 4840 in an acre. When the glass had stood a quarter 
of an hour, he wiped it with a piece of muslin, the weight of which had been 
previously ascertained. When the glass had been wiped dry, he again weighed 
the muslin, and lound that its weight had been increased six grains, by the water 
collected from twenty square inches of earth ; a quantity equal to 1600 gallons, 
from an acre, in twenty-four hours. Another experiment, after rain had fallen, 
gave a much larger quantity. [See Illustration '2d, page 12.] 

When the atmosphere is colder than the earth, tie vapor, which arises from the 
ground, or a body of water, is condensed and becomes visible. This is the way 
that fog is produced. When the earth is colder than the atmosphere, the moisture 
in the atmosphere condenses in the form of dew, on the ground, or other surfaces. 

Clouds are nothing more than vapor, condensed by the cold of the upper regions 
of the atmosphere. 

Rain is produced by the sudden cooling of large quantities of watery vapor. 

Snow and hail are produced in a similar manner, and differ from rain, only, by 
the degree of cold which produces them; 
f This is the reason why this kind of pump is sometimes called ' the lifting pump.' 

For what is the hygrometer used ? Of what kind of substances may it be con- 
structed? What experiment is given in the note to show the quantity of moisture 
raised from the ground by the heat of the sun ? How is fog produced .' What are 
clouds ? How is rain produced ? How aro snow and hail produced ? 259. By what 
means is wa'er raised in the common pump? How is the pressure removed.' 
What fiff. represents the common pump '. Explain it. Why is this sometimes call- 
ed the lifting pump » 



PNEUMATICS. 



95 



Fi?. 84. 



um is again formed below, which is immediately 
filled by the water rushing through the lower valve 
Y. In" this manner the body of the pump is filled 
with water, until it reaches the spout S, where it 
runs out in an interrupted stream.* 

260. Water can be raised by the com- 
mon pump only about 32 feet, because the 
weight or pressure of the atmosphere is 
equal to the weight of a column of water 
of that height only. 

261. The forcing pump differs from 
the common pump in having a forcing 
power added to raise the water to any de- 
sired height. 

Fig. 85 represents the forcing pump. The body 
and lower valve V are similar to those in the com- 
mon pump. The piston 
P has no valve, but is 
solid ; when, therefore, 
the vacuum is produc- A 
ed above the lower 
valve, the water on 

the descent of the piston is forced 

through the tube into the reservoir or 

air vessel R, where it compresses the 

air above it. The air, by its elasticity, 

forces the water oui through the jet J 

in a continued stream and with great 

force. It is on this principle that fire 

engines are constructed. 

Sometimes a pipe with a valve in it 

is substituted for the air vessel; the 

water is then thrown out in a continued 

stream, but not with so much force. 

262. Wind is a current of air put in motion.f 

* The handle of a common pump 13 a lever of the first kind, the shorter 
arm of which is connected with the piston."* When the handle is pressed down, the 
piston a.-cends. The valves, and the parts which contain them, are, in common 
language, called " boxes.'" 

t There are two ways in which the motion of the air may be explained . It may 
be considered as an absolute motion of the air, ratified by heat and condensed by 
cold — or ii may be only an apparent motion, caused by the superior velocity of 
the earth in its daily revolution. 



B 




Which of the mechanical powers i3 the handle of the pump? 260. How high 
can water be raised by the common pump .' Why J 261. How does the forcing 
pump differ from the common pump; What figure represents the forcing pump I 
Explain it. 262. What is wind ! In what two ways may the motion of the 
air be explained? 



96 NATURAL PHILOSOPHY. 

Explanation. When any portion of the atmosphere is heated, it 
becomes ratified, its specific gravity is diminished, and it conse- 
quently rises. The adjacent portions immediately rush into its 
place to restore the equilibrium. This motion produces a cur- 
rent which rushes into the rarified spot from all directions. 
This is what we call wind. The portions north of the rarified spot 
rush downwards, producing a North wind; those to the south rush 
upwards, producing a South wind; while those to the East and.West, 
in like manner, form currents moving in opposite directions. At 
the rarified spot, agitated as it is by winds from all directions, tur- 
bulent and boisterous weather, whirlwinds, hurricanes, rain, thun- 
der and lightning prevail. This kind of weather occurs most fre- 
quently in the torrid zone, where the heat is greatest. The air be- 
ing more rarified there than in any other part of the globe, is light- 
er, and, consequently, ascends; that about the polar regions is 
continually flowing from the poles to the equator, to restore the 
equilibrium ; while the air rising from the equator flows in an upper 
current towards the poles, so that the polar regions may not be ex- 
hausted.* A regular east wind prevails about the equator, caused 
by the rarefaction of the air produced by the sun in his daily course 
from east to west. This wind, combining with that from the poles, 
causes a constant north-east wind, for about thirty degrees north of 
the equator, and a south-east wind at the same distance south of the 
equator. 

*From what has now been said, it appears that there is a circulation of air in 
the atmosphere ; the air in the lower strata flowing from the poles to the equator ; 
and in the upper strata flowing back from the equator to the poles. It may here 
be remarked that the periodical winds are more regular at sea than on the land ; 
and the reason of this is that the land reflects into the atmosphere a much greater 
quantity of the sun's rays than the water ; therefore, that part of the atmosphere 
which is over the land is more heated and rarified than that which is over the sea: 
this occasions the wind to set in upon the land, as we find it regularly does on the 
coast of Guinea and other countries in the torrid zone. There are certain winds 
called trade-winds, the theory of which may easily be explained, on the principle of 
rarefaction, aflvcted as it is by the relative position of the different parts of the 
earth with the sun, at different seasons of the year and at various parts of the day. 
A knowledge of the laws, by which these winds are controlled, is of importance 
to the mariner. When the position of the sun, with respect to the different 
positions of the earth, at the different seasons of the year is understood, it will be 
seen that they all depend upon the same principle. The reason that the wind 
generally subsides at the going down of the sun is, that the rarefaction of the air, 
in the particular spot which produces the wind, diminishes as the sun declines, and 
consequently the force of the wind abates. The great variety of winds in the 
temperate zones is thus explained. The air is an exceedingly elastic fluid, yield- 
ing to the slightest pressure ; the agitations in it, therefore, caused by the regular 
winds, whose causos have been explained, must extend every way to a great dis- 
tance ; and the air, therefore, in all climates will suffer more or less perturbation, 

Explain the manner in which the air is put in motion. How are the north, 
south, cast, and west winds produced? What kind of weather prevails at the 
rarified spot? Where does this kind of weather occur most frequently? What 
causes a regular east wind to prevail about the equator I Why are the periodical 
winds more regular at sea than on the land ? How would you account for the 
winds called trade-winds monsoons, &c. What is the reason that the wind gen- 
erally subsides at the going down of the sun? How can the great variety of 
winds, in the temperate zones, be explained I 



ACOUSTICS. 97 

SECTION XII. 

Acoustics. 



263. Acoustics is the science which treats of the na- 
ture and laws of sound. It includes the theory of mu- 
sical concord or harmony. 

264. Sound is caused by a tremulous or vibratory 
motion of the air. 

Illustration. If a bell be rung under an exhausted receiver, no 
sound can be heard from it ; but when the air is admitted to sur- 
round the bell, the vibrations immediately produce sound. 

Again, if the experiment be made by inclosing the bell in a small 
receiver, full of air, and placing that under another receiver, from 
which the air can be withdrawn, though the bell, when struck, 
must then produce sound, as usual, yet it will not be heard if the 
outer receiver be well exhausted, and care be taken to prevent the 
vibrations from being communicated through any solid part of the 
apparatus ; because there is no medium through which the vibra- 
tions of the bell, in the smaller receiver, can be communicated to 
the ear. 

265. Sounds are louder when the air surrounding the 
sonorous body is dense, than when it is in a rarified 
state. 

For this reason the sound of a bell is louder in cold than in warm 
weather ; and sound of any kind is transmitted to a greater distance 
in cold, clear weather, than in a warm sultry day. On the tops of 
mountains, &c. where the air is rare, the human voice can be heard 
only at the distance of a few rods ; and the firing of a gun produ- 
ces a sound scarcely louder than the cracking of a whip. 

according to the situation of the country, the position of mountains, valleys, and 
a variety of other causes. Hence every climate must be liable to variable winds. 
The quality of winds is affected by the countries over which they puss ; and they 
are sometimes rendered pestilential by the heat of deserts, or the putrid exhala- 
tions of marshes and lakes. Thus, from the deserts of Africa, Arabia, and the 
neighboring countries, a hot wind blows, called Samiel or Simoom, which some- 
times produces instant death. A similar wind blows from the desert of Sahara, 
upon the western coast of Africa, called the Harmattan, producing a dryness 
and heat which is almost insupportable, scorching like the blasts of a furnace. 



By what is the quality of winds affected >. What is that wind called which 
blows from the deserts of Arabia and Africa >. What is that called which blow3 
from the desert of Sahara ? 26M. What is that science called which treats of the 
nature and laws of sound ? What does it include; 264. What causes sound? 
What illustrations are given to prove this? 265. In what proportion are sounds 
loud or faint ? Why does a bell sound louder in cold than in warm weather I Why 
is sound fainter on the top of a mountain, than nearer the surface of the earth I 



98 NATURAL PHILOSOPHY. 

266. Sonorous bodies are those which produce clear, 
distinct, regular and durable sounds, such as a bell, a 
drum, wind instruments, musical strings and glasses. 
These vibrations can be communicated to a distance 
not only through the air, but also through liquids and 
solid bodies. 

267. Bodies owe their sonorous property to their 
elasticity.* 

263. The sound produced by a musical string is caus- 
ed by its vibrations ; and the height or depth of the 
tone depends upon the rapidity of these vibrations. 
Long strings vibrate with less rapidity than short ones, 
and for this reason the low tones in a musical instru- 
ment proceed from the long strings, and the high tones 
from the short ones. 

Illustration. Fig. 86, A B represents a musical string. If it be 

Fig- 86. drawn up to 

- G, its elasticity 

>p ^ ""**" ,_ will not only 

>-**' 3E -• •.I**'***, carry it back 

„<^-'_lT-~ "^ .«.-.**' s »^>* again, but will 

give it a mo- 
mentum which 
will carry it 
to H, from 



*. --^ '--■ D ""'->''„ 

""31 whence it will 

successively return to T, F, C, D, &c. until the resistance of the 
air entirely destroys its motion. 

The vibrations of a sonorous body gives a tremulous motion to 
the air around it, similar to the motion communicated to smooth 
water when a stone is thrown into it. 

269. The science of harmony is founded on the rela- 
tion which the vibrations of sonorous bodies have to 
each other. Thus when the vibrations of one string are 
double those of another, the chord of an octave is pro- 
duced. If the vibrations of two strings are as two to 

* Although it is undoubtedly the case that all "sonorous bodies are elastic, it 
is not to be inferred that all elastic bodies are sonorous. 



266. What are sonorous bodies? 267. To what do sonorous bodies owe their 
sonorous property? Are all elastic bodies sonorous? 268. What causes the 
Bound produced by a musical string ! Upon what does the height and depth of 
the tone depend ? Which strings, in a musical instrument, produce the low tones .' 
Why! Explain fig. 86. 269. Upon what is the science of harmony founded; ilow 
la trie chord, of an octave produced .' 



acoustics. yy 

three, the chord of a fifth is produced.* When the vibra- 
tions of two strings frequently coincide, they produce 
a musical chord; and when the coincidence of the vi- 
brations is seldom, a discord is produced. 

270. The quality of the sound produced by strings 
depends upon their length, their thickness or weight, 
and their degree of tension. The quality of the sound 
produced by wind instruments depends upon their size, 
their length, and hollow diameters. Long and large 
strings, when loose, produce the lowest tones; but dif- 
ferent tones may be produced from the same string, 
according to the degree of tension, or the tightness 
with which it is drawn. Large wind instruments, also, 
produce the lowest tones; and different tones may be 
produced from the same instrument, according to the 
distance of the aperture, for the escape of the wind, 
from the aperture where it enters ; or, which is the 
same thing, the length of that portion of the instru- 
ment which is struck by the air. [See Note to No. "264.) 
271. The quality of the sound of all musical instru- 
ments is affected, in some degree, by the changes in 
the temperature and specific gravity of the atmosphere, 
or the air in the room. As heat expands and cold con- 
tracts the materials of which the instrument is made, 
it follows that the strings will have a greater decree of 
tension, and that pipes and other wind instruments will 
be contracted, or shortened in cold weather. For this 
reason most musical instruments are higher in tone, 
(or sharper,) in winter, or cold weather, and lower in 
tone, (or more flat) in summer, or in warm weather. 

* When music is made by the use of strings the air is struck by the body, and 
the sound is caused by the vibrations: when il is made by pipes, the body is s;ruck 
by tlie air ; but as action and reaction are equal, the effect is the same in both 
cases. 



How is the chord of a fifth produced ! How i« a musical chord produced? A 
discord? 270. Upon what does the quality of the sound, produced by s» rings, de- 
pend; Upon what does that produced by wind ins f ruments depend. 3 What 
strings produce the lowest tones I How may different tones be produced from the 
same string? How may different tone? be produced from the same wind instru- 
ment; 271. What, in some degree, affects the quality of the sound of all mu- 
sical insirumeius ? What effecr has heat and cold on the materials of which the 
instrument is made? What follows from this i Why are most musical instru- 
ments higher in tone, or sharper, in cold weather ? 



100 NATURAL PHILOSOPHY. 

272. Sound is communicated more rapidly and with 
greater power through solid bodies, than through the 
air, or fluids. It is conducted by water about four 
times quicker than by air, and by solids about twice as 
rapidly as by water. 

If a person lay his head on a long piece of timber, he can hear 
the scratch of a pin at the other end, while it could not be heard 
through the air. 

By placing the ear against a long, dry, brick wall, and causing a 
person to strike it once with a hammer, the sound will be heard 
twice, because the wall will convey it with greater rapidity than the 
air, though each will bring it to the ear. 

The Stethescope is an instrument depending on the power of sol- 
id bodies to convey sound. It consists of a wooden cylinder, one 
end of which is applied firmly to the breast, while the other end is 
brought to the ear. By this means the action of the lungs and other 
internal parts of the human body may be distinctly heard. The 
instrument, therefore, becomes useful in the hands of a skilful phy- 
sician to ascertain the state of the internal organs. 

273. Sound, passing through the air, moves at the 
rate of 1142 feet in a second of time. This is the case 
with all kinds of sound. The softest whisper flies as 
fast as the loudest thunder, and the force or direction 
of the wind makes but slight difference in its velocity. 

This uniform velocity of sound enables us to determine the dis- 
tance of an object from which it proceeds. If, for instance, the light 
of a gun, fired at sea, is seen a half of a minute before the report 
is heard, the vessel must be at the distance of six miles and a half. 
In the same manner the distance of a thunder cloud may be ascer- 
tained by counting the seconds between the appearance of the light- 
ning and the noise of the thunder, and multiplying them by 1142' 
feet. 

274. An echo is produced by the vibrations of the 
air meeting a hard and regular surface, such as a wall, 
a rock, mountain, &c. and being reflected back to the 
ear, thus producing the same sound a second and 
sometimes a third and fourth time.* 

* From this it is evident tliat no echo can be heard at sea, or on an extensive 
plain ; because there is no object there to reflect the sound. An echo is heard only 

272, Through which is sound communicated more rapidly, and with greater 
power, through solid bodies or the air.' How fast is it conducted by water? How 
fast by solids? What examples are given to show that sound is communicated 
more rapidly through solid bodies than the air or fluids ? What is a stethescope ? 
Of what does it consist? For what is it used ? 273 How fast does sound move? 
Does the force or direction of the wind make any difference in its velocity? What 
advantage results from this uniform velocity of sound ? How can the distance 
of a thunder cloud be ascertained? 274. How is an echo produced.' 



ACOUSTICS. 101 

275. Speaking trumpets are constructed on the prin- 
ciple of the reflection of sound. 

The voice, instead of being diffused in the open air, is confined 
within the trumpet; and the vibrations which spread and fall 
against the sides of the instrument are reflected according to the 
angle of incidence, and fall in the direction of the vibrations which 
proceed straight forward. The whole of the vibrations are thus 
collected into a focus ; and if the ear be situated in or near that spot, 
the sound is prodigiously increased. 

Hearing trumpets, or the trumpets used by deaf persons, are, also, 
■constructed on the same principle ; but as the voice enters the large 
end of the trumpet, instead of the small one, it is not so much con- 
fined, nor so much increased. 

The musical instrument called the trumpet acts, also, on the same 
principle with the speaking trumpet, so far as its form tends to in- 
crease the sound.* 

276. Sound, like light, after it has been reflected 
from several places may be collected into one point, 
as a focus, where it will be more audible than in any 
other part ; and on this principle whispering galleries 
may be constructed. 

The famous whispering gallery in the dome of St. Paul's church, 
in London, is constructed on this principle. Persons at very remote 
parts of the building can carry on a conversation in a soft whisper, 

when a person stands in such a situation as to hear both the original and the re- 
flected sound. The pupil will doubtless recollect what has been said in mechan- 
ics witb respect to the angleg of incidence and reflection. Sound (as well as light, 
as will be explained under the head of optic) is communicated and reflected by 
the same law, namely, that the angles of incidence and reflection are always 
equal. It is not difficult, therefore, to ascertain the direction in which sound will 
proceed, whether it be direct or reflected. It is related of Dionysius, the tyrant 
of Sicily, that he had a dungeon (called the ear of Dionysius) in which the root 
was so constructed as to collect the words and even the whispers of the prisoners 
confined therein, and direct them along a hidden conductor to the place where he 
sat to listen ; and thus he became acquainted with the most secret expressions of 
his unhappy victims. 

* The smooth and polished surface of (he interior parts of certain kind of shells, 
particularly if they are spiral or undulating, fit them to collect and reflect the 
various sounds which are taking place in the vicinity. Hence (he Cyprias, the 
Nautilus, and some other shells, when held near or in the ear, give a continued 
sound which resembles the roar of the distant ocean. 



Whv cannot an echo be heard at. sea or on an extensive plain.' How must a 
person stand in order to hear an echo ? By what law is sound communicated and 
reflected! What anecdote is related of Dionysius >. 275. Upon what principle 
are speaking trumpets constructed ; Explain the manner in which the air is re- 
flected. Upon what principle are hearing trumpets constructed? How far does 
the musical instrument, called the trumpet, act upon the principle of the speak- 
ing trumpet .' How can the continued sound, given by some shells when held near 
the ear, be explained ? 276. Upon what principle may whispering galleries be 
constructed ? 

10 



102 NATURAL PHILOSOPHY. 

which will be distinctly audible to one another, while others in 
the building cannot hear it ; and the ticking of a watch may be 
heard from side to side. 

277. Sounds can be conveyed to a much greater dis- 
tance through continuous tubes than through the open 
air. 

The tubes used to convey sounds are called acoustic tubes. They 
are much used in public houses, stores, counting rooms, &c to con- 
vey communications from one room to another. 

278. The quality of sound is affected by the furni- 
ture of a room, particularly the softer kinds, such as 
curtains, carpets, &c, because having little elasticity 
they present surfaces unfavorable to vibrations. 

For this reason, music always sounds better in rooms with bare 
walls, without carpets, and without curtains. For the same reason 
a crowded audience increases the difficulty of speaking. 

As a general rule, it may be stated thai plane and smooth sur- 
faces reflect sound without dispersing it, convex surfaces disperse it, 
and concave surfaces collect it. 

279. The air is a better conductor of sound when it 
is humid than when it is dry. 

A bell can be more distinctly heard just before a rain ; and sound 
is heard better in the night than in the day, because the air is gen- 
erally more damp in the night. 

The distance to which sound may be heard, depends upon vari- 
ous circumstances, on which no definite calculations can be predica- 
ted. Volcanoes, among the Andes, in South America, have been 
heard at the distance of three hundred miles — naval engagements 
have been heard two hundred, and even the watch word, " alVs 
well" pronounced by the unassisted human voice, has been heard 
from Old to New Gibraltar, a distance of twelve miles. 

280. The sound of the human voice is produced by 
the vibration of two delicate membranes, situated at 
the top of the windpipe, and between which the air 
from the lungs passes. The tones are varied from grave 
to acute, by opening or contracting the passage ; and 
they are regulated by the muscles belonging to the 
throat, by the tongue and by the cheeks. 



277. In what way can sounds be conveyed to a much greater distance than 
through the air ? What are the tubes, used to convey sounds, called? 278. Why 
do the softer kinds of furniture, in a room, affect the quality of the sound .' What 
general rule is given with regard to the reflection of sound; 279. Is the air a 
better conductor when it is humid or when it is dry? Why can a sound be heard 
better in the night than in the day .' 280. How is the sound of the human voice 
produced ,' How are the tones varied and regulated ? 



ACOUSTICS. 103 

The management of the voice depends much upon cultivation; 
and although many persons can both speak and sing with ease, and 
with great power, without much attention to its culture, yet it is 
found that those who cultivate their voices by use, acquire a degree 
of flexibility and ease in its management, which, in great measure, 
supplies the deficiency of nature.* 

281. Ventriloquism t is the art of speaking in such a 
manner as to cause the voice to appear to proceed from 
a distance. 

The art of ventriloquism was not unknown to the ancients ; and 
it is supposed by some authors that the famous responses of the or- 
acles, at Delphi, at Ephesus, &c. were delivered by persons who 
possessed this faculty. There is no doubt ihat many apparently 
wonderful pieces of deception, which, in the days of superstition and 
ignorance, were considered as little short of miracles, were perform- 
ed by means of ventriloquism. Thus houses have been made to 
appear haunted, voices have been heard from tombs, and the dead 
have been made to appear to speak, to the great dismay of the 
neighborhood, by means of this wonderful art. 

Ventriloquism is, without doubt, in great measure the gift of na- 

*The reader is referred to Dr. Rush's very valuable work on the Philosophy of 
the Human Voice, for plain and practical instructions on this subject. Dr. Bar- 
ber's Grammar of Elocution, and Parker's Progressive ExercUes in Rhetorical 
Reading, likewise contain the same instructions, in a practical form. To the 
work of Dr. Rush, botli of the latter mentioned works are largely indebted. 

f The word ventriloquism literally means u speaking from the belly,?' and it is 
so defined in Chambers' Dictionary of Arts and Sciences. The ventriloquist, by a 
singular management of the voice, seems to have it in his power '* to throw his 
voice " in any direction, so that the sound shall appear to proceed from that spot. 
The words are pronounced by the organ3 usually employed for that purpose, but 
in such a manner as to give little or no motion to the lips, the organs chiefly con- 
cerned being those of the throat and tongue. The variety of sounds which the 
human voice is capable of thus producing, is altogether beyond common belief, and, 
indeed, is truly surprising. Adepts in this art will mimic the voices of all ages 
and conditions of human life, from the smallest infant to the tremulous voice of 
tottering age ; and from the intoxicated foreign beggar to the high bred, artificial 
tones of the fashionable lady. Some will also imitate the warhling of the night- 
ingale, the loud tones of the whip-poor-will, and the scream of the peacock, with 
equal truth and facility. Nor are these arts confined to professed imitators; for in 
many villages boys may be found, who are in the habit of imitating the brawling 
and spitting of cats, in such a manner as to deceive almost every hearer. 

The human voice is also capable of imitating almost every inanimate sound. 
Thus the turning and occasional creaking of a grindstone, with the rush of the 
water — the sawing of wood — the trundling and creaking of a wheel-barrow — the 
drawing of bottle corks, and the gurgling of the flowing liquor — the sound of air 
rushing through a crevice on a wintry night, and a great variety of other noises of 
the same kind, are imitated by the voice so exactly, as to deceive any hearer who 
does not know whence they proceed. 



Upon what does the management of the voice depend ? 231. What is ventrilo- 
quism? Was this art known to the ancients? What is supposed, by some authors, 
concerning the responses at Delphi, Ephesus, &c? Is ventriloquism a natural 
gift, or an acquired one I 



104 



NATURAL PHILOSOPHY. 



ture ; but many persons can, with a little practice, utter sounds and 
pronounce words without opening the lips or moving the muscles of 
the face; and this appears to be the great secret of the art. 



SECTION XIII. 

Pyronomics, or the Laws of Heat. 

282. Pyronomics is the science which treats of the 
laws, the properties and operations of heat. 

283. Whether heat is or is not a material substance, 
is not known ; but it has been proved that the addition 
of heat to any substance produces no alteration in the 
weight of that substance. Hence it is inferred that 
heat has no weight. 

284. Though heat passes through some bodies with 
more difficulty than through others, there is no body nor 
any kind of matter which can completely arrest its 
progress. 

285. Heat is generally known by the name of Calo- 
ric. There are two kinds of heat ; or rather, heat ex- 
ists in two states, called free and latent. Free heat, or 
free caloric is that which is perceptible to the senses, 
as the heat of a fire, the heat of the sun, &c. Latent 
heat is that which exists in most kinds of substances, 
but is not perceptible to the senses, until it is brought 
out by mechanical or chemical action. Thus, when a 
piece of cold iron is hammered upon an anvil it be- 
comes intensely heated ; and when a small portion of 
sulphuric acid, or vitriol, is poured into a phial of cold 
water, the phial and the liquid immediately become 
hot. 

282. Of what does pyronomics treat? From what is it inferred that heat has 
no weight ? 283. What is stated, in No. 283, with regard to heat >. 284. Can the 
progress of heat be arrested ? 285. What is caloric ? In what two states does 
heat exist ? What is free heat .' Give some examples of free heat. 



PYRONOMICS. 105 

A further illustration of the existence of latent or concealed heat 
is given at the fireside every day. A portion of cold fuel is placed 
upon the grate or hearth, aDd a spark is applied to kindle the fire 
which warms us. It is evident that the heat given out by the fuel , 
when ignited, does not all proceed from the spark, nor can we per- 
ceive it in the fuel ; it must, therefore, have existed somewhere in 
a latent state. It is, however, the effects of free heat, or free caloric, 
which are embraced in the science of pyronomics. The subject of 
latent heat belongs more properly to the science of chemistry. 

286. The terms heat and cold, as they are generally 
used, are merely relative terms ; for a substance which 
in one person would excite the sensation of heat, 
might, at the same time, seem cold to another. 

Thus, also, to the same individual, the same thing may be made 
to appear, relatively, both warm and cold. If, for instance, a per- 
son were to hold one hand near to a warm fire, and the other on a 
cold stone, or marble slab, and then plunge both into a basin of luke- 
warm water, the liquid would appear cold to the warm hand and 
warm to the cold one. 

287. The principal effects of heat, on bodies to 
which it is applied, are three ; namely, First, heat di- 
lates or increases the extension of all bodies, whether 
solid, liquid, or in the form of air, or gas. Thus, met- 
als, wood and other substances, are expanded by the 
application of heat. Secondly, heat, when applied in 
sufficient quantity to many kinds of substances, trans- 
forms them from a solid to a fluid state. Thus, metals, 
glass and many other substances can be melted by the 
application of a sufficient degree of heat. Thirdly, 
heat, when applied in its greatest degree, destroys the 
texture of many kinds of substances by combustion. 
Thus, wood, coal, and other substances are burnt up by 
the application of heat. 

288. The sources from which heat is derived are, 
First, from the sun, in connexion with light. Secondly, 
from mechanical operations, such as friction, percus- 
sion, compression, &c. Thirdly, from a variety of 
chemical operations, especially combustion; and Fourth- 
ly, from living animals and vegetables. 

What is latent heat ? Give some examples of latent heat? 286. How are the 
terms, heat and cold generally used ? What illustration of this is given? 287. 
What are the three principal effects of heat on bodies to which it i9 applied ? Give 
an example of each effect ? 288. What are the sources from which heat is derived » 

10* 



106 NATURAL PHILOSOPHY. 

289. Heat tends to diffuse itself equally through all 
substances. 

If a heated body be placed near a cold one, the temperature of the 
former will be lowered, while that of the latter will be raised. 

290. All substances contain a certain quantity of 
heat; but, by its tendency to diffuse itself equally, and 
the difference in the power of different substances, to 
conduct it, bodies of the same absolute temperature 
appear to possess different degrees of heat. 

Thus, if the hand be successively applied to a woollen garment, a 
mahogany table, and a marble slab, all of which have stood for 
some time in the same room, the woollen garment will appear the 
warmest, and the marble slab the coldest of the three articles; but 
if a thermometer be applied to each, no difference in the tempera- 
ture will be observed. 

From this it appears that some substances conduct heat readily, 
and others with great difficulty. The reason that the marble slab 
seems the coldest, is, that marble, being a good conductor of beat, 
receives the heat from the hand so readily that the loss is instantly 
felt by the hand ; while the woollen garment, being a bad conductor 
of heat, receives the heat from the hand so slowly that the loss is 
imperceptible. 

291. The different power of receiving and conduct- 
ing heat, possessed by different substances, is the cause 
of the difference in the warmth of various substances 
used for clothing. 

Thus, woollen garments are warm garments, because they part 
slowly with the heat which they acquire from the body, and, conse- 
quently, they do not readily convey the warmth of the body to the 
air ; while, on the contrary, a linen garment is a cool one, because 
it parts with its heat readily, and as readily receives fresh heat from 
the body. It is, therefore, constantly receiving heat from the body 
and throwing it out into the air, while the woollen garment retains 
the heat which it receives, and thus encases the body with a warm 
covering. 

For a similar reason ice, in summer, is wrapped in woollen cloths. 
It is then protected from the heat of the air, and will not melt. 

292. Heat is received and conducted with the great- 
est readiness by metals. 

289. In what way does heat tend to diffuse itself? 290. Why do bodies of the 
same absolute temperature appear to possess different degrees of heat ? What il- 
lustration of this is given? What appears from this J 291. What causes the dif- 
ference in the warmth of substances used for clothing? Why are woollen gar- 
ments warm .' Why are linen ones cold? "Why is ice wrapped in woollen in sum- 
mer ! 292. By what is heat received and conducted with the greatest readiness ? 



PYRONOMICS. 107 

For this reason wooden spoons and forks are used in preference 
to silver ones, to take ice from a plate. The spoon is of the same 
temperature with all other articles in the room; and if it be of sil- 
ver, or any other metal, it readily communicates its heat to the ice 
and melts it — but wooden spoons do not so readily part with their 
heat, and will not, therefore, melt the ice so readily. 

For the same reason, the handles of tea and coffee pots are gene- 
rally made of wood; parting with their heat less readily than me- 
tallic ones, they are less likely to be inconvenient to the hand, on 
account of their heat. 

293. All bodies, whether solid, liquid, or in the form 
of gas, when violently compressed or extended, become 
warm. 

Experiment. If a piece of Indian rubber be quickly stretched 
and applied to the lip, a sensible degree of heat will be felt. An 
iron bar, by being hammered, becomes red hot ; and even water, 
when strongly compressed, gives out heat. 

When air is forcibly compressed* by driving down the piston of 
a syringe, nearly closed at the end, great heat is produced. Syring- 
es have been constructed on this principle for procuring fire, the 
heat, thus produced, being sufficient to kindle dry tinder. 

294. All substances, with regard to their capacity 
for heat, may be divided into two classes, namely, 
combustible or inflammable bodies, and incombustible 
or non-inflammable bodies. 

Vegetable Substances, charcoal, oils, most animal substances, as 
hair, wool, horn, fat, and all metallic bodies, are combustible.t 
Stones, glass, salts,. &c. are incombustible. 

* The following fact is extracted from a newspaper, published in this city, 
December 3, 1836. 

" Solid Air . The philosophers of Paris, by the aid of tremendously powerful 
apparatus, have succeeded in the consolidation of carbonic acid gas, one of the 
constituents of atmospheric air. so as to be both visible and tangible. The sub- 
stance, at a late sitting of the French Academy, was distributed to the company 
tasted and handled — and the sensation produced by its touch is described as ' the 
impression of extraordinary cold, which a solid gas produces, when returning 
fiom a state of air.' It is added, that the company were much surprised at the 
slight effect resulting to the organs of sensation from contact with a substance 
the touch of which congeals mercury and spirits of wine, and causes the ther- 
mometer to descend to 90 degrees below zero." 

f The word combustible literally means, that which can be burnt up. The pu- 
pil is referred to Must. 3, page 12, for some remarks with regard to the consump- 
tion, or rather the alteration which takes place in bodies during combustion. 



Why are wooden spoons and forks sometimes used in preference to silver ones? 
293. What instated in No. 293 ? What experiments are here related to illustrate 
this .' What is said of the air when strongly compressed ? What fact is related 
in the note ? 294. Into what classes are all substances, with regard to their ca- 
pacity for heat, divided ; What substances are combustible? What substances 
are incombustible l 



108 NATURAL PHILOSOPHY. 

295. The pyrometer is an instrument to show the ex- 
pansion of bodies on the application of heat. It consists 
of a metallic bar or wire, with an index connected with 
one extremity. On the application of the flame of a lamp, 
or heat from any other source, to any part of the bar, 
the bar expands and turns the index to show the degree 
of expansion. 

296. The most obvious and direct effect of heat on 
a body, is to increase its extension in all directions. 

Coopers, wheelwrights, and other artificers, avail themselves of 
this property in fixing iron hoops on casks, and the tires or irons on 
wheels. The hoop or tire having been heated, of course expands, 
and being adapted in that state to the cask or the wheel, as the met- 
al contracts in cooling, it clasps the parts very firmly together.* 

297. Heat not only expands metals, wood, &c. but 
also different kinds of stones, chalk, burnt brick, and 
especially glass. 

These substances must, however, be freed from moisture, other- 
wise heat, by dissipating the moisture, will occasion contraction. 

The effect of heat and cold,t in the expansion and contraction of 
glass, is an object of common observation ; for it is this expansion 
and contraction which causes so many accidents with glass articles. 
Thus, when hot water is suddenly poured into a cold glass, of any 

* From what ha3 been stated above, it will be seen that an allowance should be 
made for the alteration of the dimensions in metallic beams or supporters, caused 
by the dilatation and contraction effected by the weather. In the iron arches of 
Southwark bridge, over the Thames, the variation of the temperature of the air 
causes a difference of height, at different times, amounting to nearly an inch. A 
happy application of this principle to the mechanic arts, was made, some years 
ago, at Paris. The weight of the roof of a building, in the Conseivatory of Arts 
and Trades, had pressed outwards the side walls of the structure, and endangered 
its security. The following method was adopted to restore the perpendicular di- 
rection of the structure. Several holes were made in the walls, opposite to each 
other, through which iron bars were introduced, which, stretching across the 
building, extended beyond the outside of the walls. These bars terminated in 
screws, at each end, to which large broad nuts were attached. Each alternate 
bar was then heated by means of powerful lamps, and their lengths being thus in- 
creased, the nuts on the outside of the building were screwed up close to it, and 
the bars were suffered to cool. The powerful contraction of the bars drew the 
walls of the building closer together, and the same process being repeated, on all 
the bars, the walls were gradually and steadily restored to their upright position. 

t Cold is merely the absence of heat; or rather, more properly speaking, inferior 
degrees of heat are termed cold. 



295. What is a pyrometer? Of what does it consist ? 296. What is the most 
obvious and direct effect of heat on a body ? What follows from what has been 
stated above ? What application of this principle is related in the note? 297. 
What other substances, beside metals, wood, &e. does heat expand ? Why must the 
substances be freed from moisture ? What is aaid of the effect of heat and cord 
on glass i 



PYRONOMICS. 109 

form, the glass, if it have any thickness, will crack ; and, on the 
contrary, if cold water be poured into a heated glass vessel the same 
effect will be produced. The reason of which is this: heat makes 
its way but slowly through glass ; the inner surface, therefore, when 
the hot water is poured into it, becomes heated, and, of course, dis- 
tended before the outer surface, and the irregular expansion causes 
the vessel to break. There is less danger of fracture, therefore, 
when the glass is thin, because the heat readily penetrates it, and 
there is no irregular expansion.* 

298. The expansion caused by heat in solid and li- 
quid bodies differs in different substances; but aeriform 
fluids all expand alike, and undergo uniform degrees 
of expansion at various temperatures. 

The expansion of solid bodies depends, in some degree, on the 
cohesion of their particles ; but as gases and vapors are destitute of 
cohesion, heat operates on them without any opposing power. 

299. The density of all substances is augmented by 
cold, and diminished by heat. 

There is. one remarkable exception to this remark, and that is in 
the case of water ; which, instead of contracting, expands at the 
freezing point, or when it is frozen. This is the reason why pitch- 
ers, and other vessels, containing water and other similar fluids, are 
so often broken when the liquid freezes in them. For the same 
reason, ice floats t instead of sinking in water ; for as its density is 
diminished, its specific gravity is consequently diminished. 

* The glass chimneys, used far oil and sas burners, are often broken by being 
suddenly placed, when cold , over a hot flame. The danger of fracture may be 
prevented (it is said) by making a minute notch on the bottom of the tube, with a 
diamond. This precaution has been used in an establishment where six lamps 
were lighted every day, and not a single glass has been broken in nine years. 

f Were it not for this remarkable property of water, large ponds and lakes, ex- 
posed to intense cold, would become solid masses of ice ; for if the ice, when 
formed on the surface, were more dense (that is, more heavy) than the water be- 
low, it would sink to the bottom, and the water above, freezing in its turn, would 
also sink, until the whole body of the water would be frozen. The consequence 
would be the total destruction of all creatures, &c. in the water. But its light- 
ness causes it to continue on the surface, protecting the water below from congela- 
tion. 



When hot water is suddenly poured into a cold glass, why will the glass 
crack ? When cold water is applied to a heated glass, why will the glass crack ? 
298. Is the expansion caused by heat in solid and liquid bodies the same in all 
substances ? How do aeriform fluids differ, in this respect, from solid and liquid 
bodies ? Upon what does the expansion of solid bodies, in some degree, depend? 
Why has heat more power over gases and vapors > 299. What effect has heat 
and cold upon the density of all subs'ances? What exception is there to this re- 
mark ? Why are the vessels, containing water, and other similar fluids, so often 
broken when the liquid freezes in them > Why does ice float upon the water in- 
stead of sinking in it ? What is stated, in the note, with regard to this property 
of water » 



110 NATURAL PHILOSOPHY. 

300. Different bodies require different quantities of 
heat to raise them to the same temperature, and those 
which are heated with most difficulty retain their heat 
the longest. 

Thus oil becomes heated more speedily than water, and it like- 
wise cools more quickly. 

301. When heat is thrown upon a bright or polished 
surface it is reflected,* and the angle of reflection will 
be equal to the angle of incidence. [See note to No. 
274, page 100.] 

30*2. When a certain degree of heat is applied to 
water it converts the water into steam or vapor. The 
temperature of steam is always the same with that of 
the liquid from which it is formed, while it remains in 
contact with that liquid. When closely confined its 
elastic power is sufficient to burst the vessel in which it 
is confined. 

303. The elasticity or elastic force of steam is in- 
creased and diminished by heat and cold. The amount 
of pressure, therefore, which it will exert depends on 
the temperature at which it is formed. If the steam be 
formed from the water with great additions of heat, so 
as to increase its expansive power, it is called high 
pressure steam ; and this forms the distinction between 
high and low pressure steam engines. The great and 
peculiar property of steam, on which its mechanical 
agencies depend, is its power of creating, at one mo- 
ment, a high degree of elastic force, and losing it in- 
stantaneously by the next moment. 

* Advantage has been taken of this property of heat in the construction of a 
simple apparatus fur baking. It is a bright tin case having a cover inclined to- 
wards the fire in such a manner as to reflect the heat downwards. In this man- 
ner use is made botli of the direct heat of the fire, and the reflected heat, which 
would otherwise pass into the room. The whole apparatus, thus connected with 
the culinary department, is called, in New England, " The Connecticut baker." 

300. Can all bodies be raised to the same temperature by the same quantities of 
heat ! What bodies retain their heat the longest : 301. What becomes of the heat 
which is thrown upon a bright or polished surface? How do the angles of inci- 
dence and reflection compare with each other r 302. When is water converted 
into steam or vapor ? How dors the temperature of the steam compare with that 
of the liquid from which it is formed, while it remains in contact with that liquid? 
303. By what is the elasticity of steam increased and diminished.' Upon what 
does the amount of pressure, which steam exerts, depend ! When is it called high 
pressure steam' What is the great and peculiar properly of steam, on which its 
mechanical agencies depend .' 



PYRONOMICS. Ill 

304. The steam engine is a machine moved by the 
expansive force of steam or vapor. 

305. The steam or vapor of water occupies a space 
about 1700 times larger than water. If, therefore, the 
steam which fills the chamber of a cylinder be sudden- 
ly converted into water, it will occupy a much smaller 
space, and produce a vacuum in the cylinder. 

306. The mode in which steam is made to act is by 
causing it to raise a solid piston, accurately fitted to 
the bore of a cylinder, like that in the forcing pump. 

The piston rod rises by the impulse of expanding steam, admitted 
into the cylinder below. When the piston is thus raised, if the 
steam below it be suddenly condensed or withdrawn from under it, 
a vacuum will be formed, and the pressure of the atmosphere, on 
the piston above, will drive it down. The admission of more steam 
below it will raise it again, and thus a continued motion of the pis- 
ton, up and down, will be produced. This motion of the piston is 
communicated to wheels, levers, and other machinery, in such a 
manner as to produce the effect intended. 

This is the mode in which the engine of Newcomen and Savery, 
commonly called the atmospheric engine, was constructed. 

The celebrated Mr. James Watt introduced two important improve- 
ments into the steam engine. Observing that the cooling of the cylin- 
der by the water thrown into it to condense the steam, lessened the ex- 
pansibility of the steam ; after it had performed its office, he con- 
trived a method to withdraw the steam from the principal cylinder, 
into a condensing chamber, where it is reconverted into water, and 
conveyed back to the boiler. 

The other improvement consists in substituting the expansive 
power of steam for the atmospheric pressure. This was performed 
by admitting the steam into the cylinder above the raised piston, at 
the same moment that it is removed from below it ; and thus the 
power of steam is exerted in the descending as well as in the as- 
cending stroke of the piston ; and a much greater impetus is given 
to the machinery than by the former method. 

From the double action of the steam, above, as well as below the 
piston, and from the condensation of the steam, after it had perform- 
ed its office, this engine is called Watt's double acting condensing 
steam engine. 



304. What is the steam engine ! 305. How much larger space does steam 
occupy than water ? 306. By what mode is steam made to act ? By what 
impulse does the piston rise? What causes the piston to descend? What im- 
provement did Mr. Watt introduce into the steam engine. 



112 



NATURAL PHILOSOPHY. 



Fi<r. 87. 




IJliLStration. Fig. 87 represents that portion of the steam engine in 
which steam is made to act, and propel such machinery as may be 
connected withit> The principal parts are the boiler, the cylinder 

and its piston, the 
condenser, the air 
pump, the steam 
pipe, the eduction 
pipe, and the cis- 
tern. In this fig- 
ure A represents 
the boiler, C the 
cylinder, with H 
its piston. B, the 
steam pipe, with 
two branches* com- 
municating with 
the cylinder, the 
one above and the 
other below the piston. This pipe has two valves, F and G, which 
are opened and closed, alternately, by machinery connected with the 
piston. The steam is carried through this pipe by the valves, when 
open, to the cylinder both above and below the piston. K is the 
eduction pipe, having two branches, like the steam pipe, furnished 
with valves &c. which are opened and shut by the same machinery. 
By the eduction pipe the steam is led off from the cylinder as the 
piston ascends and descends. 

L is the condenser, and O a stop cock for the admission of cold 
water. M is the air pump. N is the cistern of cold water in which 
the condenser is immersed. R is the safety valve. When the 
valves are all open the steam issues freely from the boiler, and cir- 
culates through all the parts of the machine, expelling the air.t 
Now, the valves F and Q., being closed, and G and P remaining 
open, the steam presses upon the cylinder and forces it down. As 
it descends it draws with it the end of the working beam, which is 
attached to the piston rod J, (but which is not represented in the fig- 
ure.) To this working beam, (which is a lever of the first kind) 
bars or rods are attached, which, rising and falling with the beam 
and the piston, open the stop cock O, admitting a stream of cold wa- 
ter, which meets the steam from the cylinder and condenses it, leav- 

* The steam anJ the eduction pipes are sometimes made in forms differing from 
thos6 in the figure, and they differ much in different engines. 

f This process is called blowing out, and is heard when a steamboat is about 
starting. 



What does figure 87 represent? Wha-t^are the principal parts? What 
does A represent.' What does C represent? What does B represent: What 
does K represent ? By what is the steam led off from the cylinder? What 
does L represent ? What does O represent' What does M represent? What 
does N represent ? What does R represent .' When the valves are all open, what 
becomes of the steam ? When the valves P and Q. are closed, and G and P open, 
upon what does the steam press? What does the cylinder draw with it in its de- 
scent.' Which of the mechanical powers is this working beam? 



PYRONOMICfe. H3 

iag no force below the piston to oppose its descent. At this moment 
the rods attached to the working beam close the stop cocks, G and 
P. and open F and Q. The steam then flows in below the piston, 
and rushes from above it into the condenser, by which means the 
piston is forced up again with the same power as that with which it 
descended. Thus the steam cocks, G and P and F and &, are alter- 
nately opened and closed; the steam passing from the boiler drives 
the piston alternately upwards and downwards, and thus produces a 
regular and continued motion. This motion of the piston being 
communicated to the working beam, is by that beam communicated 
to other machinery, and thus an engine of great power is obtained. 

The air pump, M, the rod of which is connected with the work- 
ing beam, carries the water from the condenser back into the boil- 
er, by a communication represented in figure 88. 

The safety valve R is made to open when the pressure of the 
steam, within the boil-er, is too great. The steam then rushing 
through the aperture under the valve removes the danger of the 
bursting of the boiler. 

307. The steam engine* is constructed in various 
forms; the principal of which are the rngh and the low 
pressure engines ; or, as they are sometimes called, the 
non-condensing and the condensing engines. 

The non-condensing or high pressure engines differ from the low 
pressure or condensing engines in having no condenser. The steam, 
after having moved the piston, is let off into the open air. As this 
kind of engine occupies less space, and is much less complicated, it 
is generally used on rail roads. 

In the low pressure or condensing engines, the steam, after having 
moved the piston, is condensed or converted into water, and then 
conducted back into the boiler. 

* The steam engine, as it is constructed at the present day. is the result of the 
inventions and discoveties of a number of distinguished individuals, at different 
periods. Among those who have contributed to i f s present state of perfection, and 
its application to practical purposes, may be mentioned t he names of Somerset, 
the Marquis of Worcester, Savery, Ncwcomen, Fulton, and especially Mr. James 
Watt. 

To the inventive genius of Watt the engine is indebted for the condenser, the 
appendages for parallel motion, the :ipplication of the governor, and for the 
double action. Jn the words of Mr. Jeffrey, it may be added that, " By his admi- 
rable contrivances, and those of Mr. Fulion, it has become a thing alike stupen- 
dous for its force and its flexibility — for the prodigious power it can exert, aud 
the ease and precision, and ductility with which it can be varied, distributed,and 
applied. The trunk of an elephant, that can pick up a pin, or rend an oak, is as 
nothing to it. It can engrave a seal, and crush masses of obdurate metal before 
it — draw out, without breaking, a thread as fine as gossamer, and lift up a ship of 
war like a bauble in tho air. [t can embroider muslin and forge anchors — cut 
steel into ribands, and impel loaded vessels against the fury of the winds and 
waves. 

What are attached to this working beam i What is their use? What becomes of 
the steam when the stop cocks G and P are closed and F aud Q are open '. How is 
the regular and continued motion produced ? To what is this motion of the piston 
communicated > What is the use of the air pump M ? For what is the safetv valve 
R used ! 307. What are the principal forms in which the steam engine is construct- 
ed ? How do they differ from each other ? What becomes of the steam after bavin* 
moved the piston in the non-condensing engines ! What kind of engines is o e „e° 
rally used on rail roads ! M hat becomes of the steam after having moved tho pis- 
ton in the condensing engines ? 

11 



114 NATURAL PHILOSOPHY. 

Fig. 88 represents Watt's double acting, condensing, steam en- 
gine, in which A represents the boiler, containing a large quantity 
of water, which is constantly replaced as fast as portions are con- 
verted into steam. B is the steam pipe, conveying the steam to the 
cylinder, having a steam cock b to admit or exclude the steam at 
pleasure. 

C is the cylinder, surrounded by the jacket c c, a space kept 
constantly supplied with hot steam, in order to keep the cylin- 
der frombeing cooled by the external air. D is the eduction pipe, 
communicating between the cylinder and the condenser. E is the 
condenser, with a valve e called the injection cock, admitting a jet 
of cold water, which meets the steam the instant that the steam en- 
ters the condenser. F is the air pump, which is a common suction 
pump, but is here called the air pump because it removes from the 
condenser not only the water, but also the air, and the steam that es- 
capes condensation. G G is a cold water cistern, which surrounds 
the condenser and supplies it with cold water, being filled by the 
cold water pump which is represented by H. I is the hot well, con- 
taining water from the condenser. K is the hot water pump, which 
conveys back the water of condensation from the hot well to the 
boiler. 

L L are levers, which open and shut the valves in the chan- 
nel between the steam pipe, cylinder, eduction pipe, and condenser ; 
which levers are raised or depressed by projections attached to the 
piston rod of the condenser. M M is an apparatus for changing, 
the circular motion of the working, beam into parallel motion, so 
that the piston rods are made to move in a straight line. N N 
is the working beam, which being moved by the rising and falling 
of the piston, attached to one end, communicates motion to the 
fly wheel by means of the crank P, and from the fly wheel the mo- 
tion is communicated by bands, wheels or levers to the other parts 
of the machinery. O O is the governor. [See Jig. 45, No, 193.] 

The governor "being connected with the fly wheel, is made to par- 
ticipate the common motion of the engine, and the balls will remain 
at a constant distance from the perpendicular shaft, so long as the 
motion of the engine is uniform ; but whenever the engine moves 
faster than usual, the balls will recede farther from the shaft, and 
by raising a valve connected with the boiler, will let ofT such a por- 
tion of the force as to reduce the speed to the rate required. 

The steam engine, thus constructed, is applied to boats to turn 
wheels having paddles attached to their circumference, which an- 
swer the purpose of oars. It is used, also, in workshops, factories, 
&c; and different directions and velocities may be given to the mo- 
tion produced by the action of the steam on the piston, by connect- 
ing the piston on the beam with wheels, axles, and levers, as repre- 
sented in numbers 171 to 180, page 52. 

What does fis- 88 represent? What does A represent? What does B represent ? 
What does C represent! What does D represent! What does E represent? What 
does F represent.' What does GG represent! What does J represent? What 
does K represent ? What does L L represent ! What does M M represent f What 
does N i\ represent What does O O represent ? What is said of the governor i 



116 NATURAL PHILOSOPHY. 

308. The Locomotive Engine is a high pressure steam 
engine, mounted on wheels, and used to draw loads on 
a rail road, or other level roads. It is usually accom- 
panied by a large wagon, called a tender, in which the 
wood and water, used by the engine, are carried. 

Fig. 89 represents a side view of the internal construction of a 
locomotive steam engine ; in which, F represents the fire box, or 
place where the fire is kept ; D the door through which the fuel is 
introduced. G one of the bars of the grate at the bottom. The 
spaces marked B are the interior of the boiler, in which the water 
stands at the height indicated by the dotted line. The boiler is clos- 
ed on all sides ; all its openings being guarded by valves. The tubes 
marked e e conduct the smoke and flame of the fuel through the 
boiler to the chimney C C, serving, at the same time r to communi- 
cate the heat to the remotest part of the boiler. By this arrange- 
ment none of the heat is lost r as these tubes are all surrounded by 
the water. S S S is the steam pipe, open at the top B S, having a 
steam-tight cock or regulator V, which is opened and shut by the 
crank H, extending outside of the boiler,, and which is managed by 
the engineer.* 

The operation of the machine is as follows : the steam being gen- 
erated in great abundance in the boiler, and being unable to escape 
out of it, acquires a considerable degree of elastic force. If at that 
moment the cock V is opened, by the handle H, the steam penetrat- 
ing into the tube S at the top near X, and in the direction of the ar- 
rows, passes through the tube and the valve V, and enters the valve 
box i. There, a sliding valve o o, which moves at the same time 
with the machine, opens for the steam a communication successive- 
ly with each end of the cylinder. Thus, in the figure, the entrance 
on the left hand of the sliding valve is represented as being open, 
and the steam follows in the direction of the dotted line into the 
cylinder, where its expansive force will move the piston in the di- 
rection of the arrow. The steam or air on the other side of the 
piston passes out in the direction of the dotted line to «, which com- 
municates with the tube t t, from which it passes into the chimney 
C, and thence into the open air. The sliding valve o o now moves 
and leaves the right hand aperture open, while it clones the one on 
the left. The steam then draws the piston back, and that portion of 
the steam on the left of the piston, having performed its office, pass- 
es out of the aperture «, an opening to which is made by the new 
position of the sliding valve. Thus, the sliding valve opening a 
communication, alternately, with each side of the piston, the steam 
is admitted on both sides of the piston, and having performed its 
office, it passes through the aperture u to the tube 1 1 and the chim- 
ney C, and from thence into the open air. 
Motion being thus given to the piston, it is communicated, by 

* This cock is not seen in the figure because it is in the inside of the tube. The 
figure represents the outside. 

Describe the locomotive Pteam engine. In the 89lh figure what do F D and G 
represent? What do the following references respectively represent, namely, S 
S S ; BBRi eeeel C C .' oo>. ul H i 1 1 .' P ? RGKi 



HI '°i lttt|1 e P j3 




ll* 



118 



NATURAL PHILOSOPHY. 



means of the rod R and the beam G, to the crank K ; which, being^ 
connected with the axle of the wheel, causes it to turn, and thus 
moves the machine. 

Thus constructed, and placed on a railroad, the locomotive steam 
engine is advantageously used as a substitute for horse power, for 
drawing heavy loads. 

The apparatus of safety valves and other appliances, for the man- 
agement of the power produced by the machine, are the same in 
principle, though different in form from those used in other steam 
engines ; for a particular description of which, the student is referred 
to practical treatises upon the subject.* 

309. Heat is propagated in two ways, namely, by 
conduction and by radiation. Heat is propagated by 
conduction when it passes from one substance to anoth- 
er in contact with it. Heat is propagated by radiation 
when it passes through the air or any other elastic 
fluid. Different bodies conduct heat with different de- 
grees of facility. The metals are the best conductors, 
and among metals silver is the best conductor. 

For this reason any liquid may be heated in a silver vessel more 
readily than in any other of the same thickness. The metals stand 
in the following order, with respect to their conductingpower, name- 
ly, silver, gold, tin, copper, platina, steel, iron and lead. 

It is on account of the conducting power of metalst that the han- 
dles of metal tea pots, and coffee pots, are commonly made of wood ; 
since, if the}" were made of metal, they would become too hot to be 
graspedby the hand, soon after the vessel is filled with heated fluid. 
Wood conducts heat very imperfectly. It may be held by the fin- 
gers very near the part which is burning, or red hot. Animal and 
vegetable substances, of a loose texture, such as fur, wool, cotton, 
&c conduct heat very imperfectly: hence their efficacy in preserv- 
ing the warmth of the body. 

310. Heat is reflected from bright surfaces ; while 
black or dark colored bodies absorb the heat that falls 
on them. 

*In"j? Practical Treatise on Locomotive Engines wpon Railways," by the 
Chevalier F. M. Gf. DePambour. the reader will find a particular description of 
all the parts of the locomotive engine. 

t Metals, on account of their conductin? power, cannot be handled when raised 
to a temperature above 121) degrees of Fahrenheit. Water becomes scalding hot 
at 150 deg., but air, heated far beyond the temperature of boiling water, may be 
applied to the skin without much pain. Sir Joseph Banks, with several other 

309. In what two ways is heat propagated > When is it propagated by conduc- 
tion ? When is it propagated by radiation .' Do all bodies conduct heat with 
the same degree of facility' What bodies are the best conductors? In what 
order do I he metals stand with respect to their conducting power? Is wood a 
good conductor of heat ? Why are wool, fur, &c. so efficacious in preserving the 
warmth of the body ? What is related, in the note, with regard to the conducting 
power of heat ? 310. What bodies reflect Vhe heat I What bodies absorb the 
htat; 



OPTICS. 119 

This is the reason why the bright brass andirons, or any other 
bright substances, placed near a hot fire,seldom become heated ; while 
other dark substances, further removed from the fire, become too 
hot for the hand. 

Snow or ice will melt under a piece of black cloth, when it will 
remain perfectly solid under a white one. The farmers, in some of 
the mountainous parts of Europe, are accustomed to spread black 
earth, or soot, over the snow, in the "spring, to hasten its melting, 
and enable them to commence ploughing early. 



SECTION XIV. 

Optics. 

31-1. Optics is the science that treats of light, col- 
ors and vision, or sight. 

312. The science of optics divides all substances 
into the following classes ; namely, luminous, transpa- 
rent, and translucent ; reflecting, refracting and opaque, 

313. Luminous bodies are those which shine by their 
own light; that is, by light proceeding from their own 
substance ; such as the sun, the stars, a burning lamp, 
or a fire. 

314. Transparent substances are those which allow 
light to pass through them freely, so that objects can be 
distinctly seen through them ; as glass, water, air, &c. 

315. Translucent bodies are those which permit a 
portion of light to pass through them ; but render the 
object behind them indistinct ; as horn, oiled paper, 
colored glass, &c. 

gentlemen, remained some time in a room when the heat was 52 degrees above the 
boiling point — but, though they could bear the contact of the heated air, they 
could not touch any metallic substance, as their watch chains, money, &c. Eggs, 
placed on a tin frame, were roasted hard in twenty minutes ; and a beef steak 
was overdone in thirty three minutes. 

Chantrey, the celebrated sculptor, has an oven which he uses for drying his 
plaster cuts and moulds. The thermometer generally stands at 300 deg. in it, yet 
the workmen enter, and remain in it some minutes, without difficulty ; but a gen- 
tleman once entering it with a pair of silver-mounted spectacles on, had his face 
burnt when the metal came in contact with the skin. 

Why do bright bodies, when placed near a hot fire, seldom become heated I 
311. Of what does optics treat ? 312. Into what classes does the science of op- 
tics divide all substances i 313. What are luminous bodies ? Give an example 
of a luminous body ? 314. What are transparent bodies ! Give an example of a 
transparent body. 315 What are translucent bodies I Give an example of a. 
translucent body. 



120 NATURAL PHILOSOPHY. 

316. Reflecting substances are those which do not 
permit light to pass through them ; but throw it off in a 
direction more or less oblique, according as it falls on 
the reflecting surface ; as polished steel, looking glass- 
es, polished metal, &c. 

317. Refracting substances are those which turn the 
light from its course, in its passage through them ; and 
opaque substances are those which permit no light to 
pass through them ; as metals, wood, &c. 

318. It is not known what light is. Sir Isaac New- 
ton supposed it to consist of exceedingly small parti- 
cles, moving from luminous bodies ; others think that 
it consists of the undulations of an elastic medium, 
which fills all space, and which produces the sensation 
of light to the eye, in the same manner as the vibrations 
of the air produce the sensation of sound to the ear. 

Which of these opinions is the more correct, it is not possible, nor 
is it important, to decide. The laws relating to light, and all the 
phenomena, which are explained in the science of optics, are equal- 
ly consistent with either opinion. 

319. A ray of light is a single line of light proceed- 
ing from a luminous body. 

320. Rays of light are said to diverge when they sep- 
arate more widely, as they proceed from the luminous 
body. 

e>:» on V 

Fig 90 represents the rays of light diverg- 
ing as they proceed from the luminous 
body, from F to D. 

321. Rays of light are called converging when they 
approach each other. The point at which converging 
rays meet is called the focus. 

Fig. 91. 

Fig. 91 represents converging rays of 
light, and the point F is the focus. 



316. What are reflecting substances ? Give an example of a reflecting body > 
317. What are refracting substances ? What are opaque substances J 318. What 
is light.' What did Sir Isaac Newton suppose it to be? What other opinion* 
have been formed concerning it ! 319. What is a ray of light ? What is a beam 
of light? 320. When are rays of light said to diverge .' Whatdoe9 rig. 90 repre- 
sent I 321. When are rays of light called converging ; What is the point,, afc 
which converging rays meet, called ? 



Fig. 90. 





OPTICS. 121 

322. A beam of light consists 
of many rays running in paral- 
lel lines. Fig. 92 represents a v 
beam of light. A pencil of light «/l\W 

is a collection of diverging or converging rays. 

323. A medium is any substance, solid or fluid, 
through which light can pass ; as water, glass, air, &c. 

324. The rays of light which proceed from terres- 
trial bodies, proceed in a diverging manner, until they 
meet with some refracting substance ; but the rays of 
the sun diverge so little, on account of the immense 
distance of that luminary, that they are considered 
parallel. 

325. Light, when proceeding from the sun, or any 
other luminous body, is projected forward in straight 
lines in every possible direction. It moves with a ra- 
pidity but little short of 200,000 miles in a second of 
time. 

326. Every point of a luminous body is a centre, from 
which light radiates in every direction. Rays, pro- 
ceeding from different bodies, cross each other without 
interfering. 

327. Light is governed by the laws of motion, but is 
not influenced by those of gravity ; or, in other words, 
it has no weight. Thus, when it falls upon any surface, 
the same law applies to it as that which governs all 
bodies, (and this may be considered as one of the funda- 
mental laws of optics, as well as of mechanics,) namely, 
that the angle of incidence is always equal to the an- 
gle of refraction. [See No. 121, page 32.] 

328. A shadow is the darkness produced by the in- 
tervention of an opaque body, which prevents the rays 
of light from reaching an object behind the opaque 
body. 

Shadows are of different degrees of darkness, because the light 



322. What does fig. 92 represent ? What is a pencil of light ? 323. What is a 
medium ! 324. In what manner do the rays of light proceed from terrestrial bod- 
ies > In what kind of lines do the rays of light proceed from the sun >. 325. la 
what way is light projected forward from any luminous body? With what rapid- 
ity does it move >. 326. From what point, in a luminous body, does light radiate ? 
327. By what laws is light governed? Has it any weight? 'What is one of the 
fundamental laws of optics ? 328. How is a shadow produced ? 



122 



NATURAL PHILOSOPHY. 



from other luminous bodies reaches the spot where the shadow is 
formed. Thus, if a shadow is formed when two candles are burn- 
ing in a room, that shadow will be both deeper and darker if one of 
the candles be extinguished. The darkness of a shadow is propor- 
tioned to the intensity of the light, when the shadow is produced by 
the interruption of the rays from a single luminous body.* 

329. When a luminous body is larger than an 
opaque body, the shadow of the opaque body will grad- 
ually diminish in size till it terminates in a point. The 
form of the shadow will be that of a cone. 



Fig. 93. 




Illustration. Fig. 93. A represents 
the sun, and B the moon. The sun, be- 
ing much larger than the moon, causes 
E it to cast a converging shadow, which 
terminates at E. 



330. When a luminous body is smaller than the 
opaque body, the shadow of the opaque body gradually 
increases in size, with the distance, without limit. 

Illustration. In fig. 94 E Fig. 94 

the shadow of the object, 
A, increases in size at the 
different distances, B, C, 
D, E, or. in other words, 
it constantly diverges. 

In estimating the effect 
of shadows, we" must con- 
sider the apparent not the 
real dimensions of the lu- 
minous body. The sun ap- 
pears smaller than the generality of the terrestrial objects which it il- 
lumines; all objects, therefore, which are apparently larger than the 
sun will cast a diverging or enlarged shadow. 




* A3 the degree of light and darkness can be estimated only by comparison, 
the strongest light will appear to produce the deepest shadow. Hence, a total 
eclipse of the sun occasions a more sensible darkness than midnight, because it is 
immediately contrasted with the strong light of day. 



Why are shadows of different degrees of darkness i To what is the darkness 
of a shadow proportioned, when the shadow is produced by the interruption of the 
rays from a single luminous body ? 329. What is said of the shadow of the 
opaque body, when the luminous body is tho larger ? Explain fig. 93. 330. What 
is said of the shadow of the opaque body, when the luminous body is the smaller? 
Explain fig. 94. What dimensions of the luminous body must we consider in esti- 
mating the effect of shadows \ Why do objects which are illumined by the sun, 
and which are really smaller than the sun, cast a diverging or enlarged shadow? 




OPTICS. 123 

331. When several luminous bodies shine upon the 
same object, each one will produce a shadow. 

Fig. 95. 

Fig. 95 represents a ball A 
illuminated by the three can- 
dles, B C and D. The light 

-d B produces the shadow b, the 
light C,the shadow c, and the 
light D, the shadow d; but as 

C the light from each of the 
candles shines upon all the 
shadows, except its own, the 

D shadows will be faint. 



332. When rays of light fall upon an opaque body, 
which they cannot pass, part of them are absorbed, 
and part are reflected, and rebound back, like an elas- 
tic ball which is thrown against a wall. By the reflec- 
tion of light is meant its return or passage from a re- 
flecting substance. 

In this respect, light is governed by the same laws as those which 
relate to solid elastic bodies. 

333. When light falls perpendicularly on an opaque 
body, it is reflected back in the same line, towards the 
point whence it proceeded. If it fall obliquely, it will 
be reflected obliquely in the opposite direction ; and in 
all cases the angle of incidence * will be equal to the 
angle of reflection. This is the fundamental law of 
reflected light. [See No. 337.] 

334. Opaque objects are seen only by reflected light. 
Luminous bodies are seen by the rays of light which 
they send directly to our eyes. 

* The angles of incidence and reflection have already been explained in page 32? 
No. 121. As the law of reflected light is one of the most important in the sci- 
ence of optics, it is necessary that the pupil have a clear idea of it. He must, 
therefore, view the particles of light as so many minute balls, bounding against a 
surface, and reflected according to this law. 

331. How many shadows are produced when several luminous bodies shine upr 
on the same object > Explain fig. 95. 332. What is the consequence when rays 
of light fall upon an opaque body which they cannot pass.' What is meant by 
the reflection of light ? By what laws is light governed, in this respect .' 333. 
How is light reflected when it falls perpendicularly on an opaque body ? How is 
it reflected when it falls obliquely > How do the angles of incidence and reflection 
compare with each other ? How should every particle of light be viewed in order 
to have a clear idea of it 1 334. By what light aro opaque objects seen ? Hovr 
are luminous bodies oe«n ? 



124 NATURAL PHILOSOPHY. 

335. The intensity of light is diminished every time 
it is reflected, because all bodies have a tendency to ab- 
sorb a portion of the light which they receive. 

336. Every portion of a reflecting surface reflects 
an entire image of the luminous body shining upon it; 
but no individual can see more than one image from the 
same surface at the same time. 

When the sun or the moon shines upon a sheet of water, every 
portion of the surface reflects an entire image of the luminary ; but 
as the imas;e can be seen only by reflected rays, and the angle of re- 
flection is always equal to the angle of reflection, the image can be 
seen only in that spot where these angles meet. 

337. Objects seen by moonlight appear fainter than 
when seen by daylight, because the light by which they 
are seen has been twice reflected. 

The moon is not a luminous body, but its light is caused by the sun 
shining upon it. This light, reflected from the moon and falling 
upon any object is again reflected by that object. It suffers, therefore, 
two reflections ; and since {See No. 335.) a portion is absorbed by 
each surface that reflects it, the light must be proportionally fainter. 
In traversing the atmosphere, also, the rays, both of the* sun and 
moon, suffer diminution; for, though the pure air is a transpa- 
rent medium, which transmits the rays of light freely, it is gene- 
rally loaded with vapors and exhalations, by which some portion of 
them is absorbed. 

338. All objects are seen by means of the rays of 
light emanating or reflected from them ; and when no 
light falls upon a body it becomes invisible. 

This is the reason why none but luminous bodies can be seen in 
the dark. For the same reason, objects in the shade, or in a darkened 
room appear indistinct, while those which are exposed to a strong 
light can be clearly seen. 

339. When rays of light, proceeding from any ob- 
ject, enter a small aperture, they cross one another and 
form an inverted image of the object. 



335. Why is the intensity of light diminished every time it is reflected? 336. 
Does every portion of a reflecting surface reflect an entire image of the luminous 
body shining upon it? How many images can be seen .' When the sun or moon 
shines upon a sheet of water, why do we not see an image reflected from every por- 
tion of the surface ? 337. Why do objects, seen by moonlight, appear fainter 
than when seen by daylight ? By what light does the moonshine? What ab- 
sorbs some of the rays of light in traversing the atmosphere? 338. How are 
all objects seen ? Why can none but luminous bodies be seen in the dark? 339. 
What kind of an image is formed when rays of light, proceeding from an object, 
enter a small aperture ? 



OPTICS. 



125 





Pig. 96. Illustration. Fig. 96 represents the rays 

from an object a c entering an aperture. 
The ray from a passes down through the 
aperture to d, and the ray from c passes up to 
b, and thus these rays, crossing at the aper- 
ture, form an inverted image on the wall. 
The room in which this experiment is made 
should be darkened, and no light permitted 
to enter, excepting through the aperture. It 
then becomes a camera obscura.* 

340. The angle of vision is the angle formed at the 

eye by two lines drawn from opposite parts of an ob- 
ject. 

Fig- 97. A Fig. 97 represents 

the angle of vision. 
The line A C pro- 
ceeding from an ex- 
tremity of the object 
meets the line B C pro- 
ceeding from the op- 
posite extremity, and 
forms an angle at the 

eye, or C ; and this is the angle of vision. 
Fie. 98 represents the different angles, made by the same object, 

at different distances. From an inspection of the figure, it is evident 

that the nearer an object is to the eye, the Fi S« 98, 

wider must be the opening of the lines 

to admit the extremities of the object ; 

and, consequently, the larger the angle 

under which it is seen; and, on the c 

contrary, that objects at a distance will 

form small angles of vision. Thus, in 

this figure, the three crosses, F G, D E, 

and A B are all of the same size; but 

A B, being the most distant, subtends the smallest angle t AC B, while 

* These words signify a darkened chamber. In the future description which 
will be given of the eye, it will be seen that the camera obscura is constructed on 
the same principle as the eye. If a convex lens, (See JVo. 357,) be placed in the 
apertoire, an inverted picture, not only of a single object, but of the entire land- 
scape, will be formed on the wall. A portable camera obscura is made by ad- 
mitting the light, into a box of any size, through a convex lens, which throws the 
image upon an inclined mirror, from whence it is reflected upwards to a plate of 
ground glass. In this manner a beautiful but diminished image of the landscape, 
or of any group of objects, is presented on the plate in an erect position. 

| The apparent size of an object depends upon the size of the angle of vision. 
But we are accustomed to correct, by experience, the fallacy of appearances ; and, 
therefore, since we know that real objects do not vary in size, but that the angles 
under which we see them do vary with the distance, we are not deceived by the 

Illustrate this by fig. 96. What is a camera obscura ? How can a portable ca- 
mera obscura be made I 340. How is the an<;le of vision formed ? Explain fig. 
97. What does fig. 98 represent ? What effect has the nearness of the object 
to the eye, on the angle ? Illustrate this by the figure. 

12 




.-A 



\. 



126 NATURAL PHILOSOPHY. 

D E and F G, being nearer to the eye, situated at C, form respective- 
ly the larger angles, D C E and F C G. 

341. When an object, at any distance, does not sub- 
tend an angle of more than two seconds of a degree, 
it is invisible. 

At the distance of four miles a man of common stature will thus 
become invisible. 

342. When the velocity of a moving body does not 
exceed twenty degrees in an hour, its motion is imper- 
ceptible. 

It is for this reason that the motion of the heavenly bodies is in- 
visible, notwithstanding their immense velocity. 

Illustration. The real velocity of a body in motion round a point, 

depends on the space comprehended in a degree. The more distant 

Fig. 99. the moving body from the centre, or, in oth- 

c er words, the larger the circle which it has 

g a to describe, the larger will be the degree. In 

fig. 99 if the man at A, and the man at B 

both start together, it is manifest that A must 

move more rapidly than B, to arrive at C at 

the same time that B reaches D ; because the 

arc A C is the arc of a larger circle than the 

arc B D. But to the eye at E, the velocity 

C n p*> of both appears to be the same, because both 

E are seen under the same angle of vision. 

343. Light is said to be reflected when it is thrown 
off from the body on which it falls. 

It has already been stated (See No. 332) that when light falls upon 
an} body, part of it is absorbed and part is reflected. It remains 
now to be observed that light is reflected in the largest quantities 
from the most highly polished surfaces. Thus, although most sub- 
stances reflect it in a degree, polished metals, looking-glasses, or 

variations in the appearance of objects. Thus, a house, at a distance, appears ab- 
solutely smaller than the window through which we look at. it; otherwise we could 
not see it through the window ; but our knowledge of the leal size of the house 
prevents our alluding to its apparent magnitude. In fig. 98 it will be seen that 
the several crosses, A B, D E, P G and H I, although very different in sizo, on 
account of their different distances, subtend the same angle A C B ; they, there- 
fore, all appear to the eye to be of the same size. 

It is upon a correct observance of the angle of vision that the art of perspective 
drawing is indebted for its accuracy. 

Upon what does the apparent size of an object depend ? Why do objects ap- 
pear so large? To what is the art of perspective drawing indebted for its accuracy? 

341. How large an angle must a body subtend to be visible ? 342. When is the 
motion of a body invisible ? Why is the motion of the heavenly bodies invisible ? 
Upon what does the real velocity of a body, in motion round a point, depend ? Ex- 
plain fig. 99. Whv does the velocity of both, to an eye at E, appear to be the 
same? 343. When is light said to be reflected? What becomes of the ligh t 
which falls upon bodies ? What surfaces reflect the largest quantity of light } 



OPTICS. 127 

mirrors, &c. reflect it in so perfect a manner as to convey to our 
eyes, when situated in a proper position to receive them, perfect im- 
ages of whatever objects shine on them, either by their own, or by 
borrowed light. 

344. That part of the science of optics which relates 
to reflected light is called Catoptrics. 

345. Rays of light are reflected according to the 
same laws which regulate the motions of elastic solid 
bodies. Thus, a ray falling on a reflecting surface will 
be thrown off from that surface in such a manner that 
the angle of incidence will be equal to the angle of re- 
flection.* This is the fundamental law of catoptrics or 
reflected light. 

346. An incident ray is a ray proceeding to, or fall- 
ing on any surface ; and a reflected ray is the ray which 
proceedsjfrow any reflecting surface. 

Fig- 100. Fig. 100 is designed to show the angles of 

incidence and of reflection. In this figure, M 
A M is a mirror, or reflecting surface. P 
is a line perpendicular to the surface. I A 
represents an incident ray, falling on the 
mirror in such a manner as to form, with the 
perpendicular P, the angle I A P. This is 
called the angle of incidence. The line R 
A is to be drawn on the other side of P A 
in such a manner as to have the same incli- 
nation with P A as I A has, so that the an- 
gle R A P will be equal to I A P. The line 
R A will then show the course of the reflect- 
ed ray ; and the angle RAP will be the an- 
gle of reflection. 
From whatever surface a ray of light is reflected, whether it be 
a plain surface, a convex surface, or a concave surface, this law in- 
variably prevails ; so that if we notice the inclination of any inci- 
dent ray, and the situation of the perpendicular to the surface, on 
which it falls, we can always determine in what manner, or to what 
point it will be reflected. This law explains the reason why, when 

* The angles of incidence and reflection have already been described in page 33, 
No. 121, but as all the phenomena of reflected light depend upon the law stated 
above, and a clear idea of these angles is necessary, in order to understand the 
law, it is deemed expedient to repeat in this connexion the explanation already 
given. 



344. What is catoptrics? 345. By what laws are rays of light reflected I What 
is the fundamental law of catoptrics >. 346. What is an incident ray ? What is 
a reflected ray ? What does fig. 100 represent ? Explain the fig. Do the differ- 
ent kinds of surfaces, from which light is reflected, cause any variation from this 
rule? 




128 



NATURAL PHILOSOPHY. 



we are standing on one side of a mirror, we can see the reflection 
of objects on the opposite side of the room, but not those on the 
same side on which we are standing. It explains, also, all the appa- 
rent peculiarities of the reflection of the different kinds of mirrors. 

347. There are three kinds of mirrors used in optics, 
namely, the plain, the concave, and the convex mir- 
ror. Plain mirrors are those which have a flat sur- 
face, such as a common looking-glass; and they neither 
magnify nor diminish the image of objects reflected 
from them. 

348. Convex mirrors have a convex surface, that is, 
a surface bulging outwards; and they diminish the im- 
age of objects reflected from them. A convex mirror 
is a portion of the outside of a sphere. 

349. Concave mirrors have a concave surface, that 
is, a surface hollowing inwards; and under certain cir- 
cumstances magnify the image of objects which they 
reflect. A concave mirror is a portion of the inner 
surface of a sphere. 

Fig. ]0l. Illustration. In fig. 101 

G M N represents both a con- 

vex and a concave mirror. 
They are both a portion of 
a sphere of which O is the 
centre. The outer part of 
M N is a convex, and the 
inner part is a concave mir- 
ror. Let G B and H F be 
two incident rays falling on 
the surface of the convex 
mirror, M N, and the 
lines P B and T F rep- 
resent the perpendiculars* 
at the points where the in- 
cident rays strike the reflecting surface ;GBP and H F T will be 
the angles of incidence ; and as the angles of reflection must be 




*It is evident that the dotted lines, P B and T F are perpendicular to the cir- 
cle, and, of course, to the arc M N, because when prolonged they will meet at the 
centre O. 



How can you explain the reason, why, when standing on one side of a mirror, 
we see the reflected objects on the opposite side I 347. What are plain mirrors .» 
How do they make the image appear ? 348. What are convex mirrors? How do 
they make the image appear ? What part of a sphere is a convex mirror? 349. 
What are concave mirrors ? How do they make the image appear ? What part 
of a sphere is a concave mirror? In fig. 101, which part of the sphere represents 
a convex mirror ? Which part a concave mirror } Explain the fig. 



OPTICS. 129 

equal to them, and on the other side of the perpendiculars, it is ev- 
ident that the incident rays will be reflected in the lines B A and 
F E, that is, that the angles of reflection, P B A and T F E will be 
equal to the angles of incidence. Now, if the rays G B and H F 
proceed from the extremities of an object, it is evident that the dot- 
ted line G H will be the length of the object ; but the length of the 
image is represented by the dotted line A E which is much shorter 
than G H ; from which it is evident that the convex mirror reflects 
a diminished image of an object. 

In a similar manner, it may be shown that concave mirrors, un- 
der certain circumstances, present magnified* images of objects re- 
flected by them. 

Concave mirrors have the peculiar property of forming images in 
the air. The mirror and the object being concealed behind a screen 
or a wall, and the object being strongly illuminated, the rays from 
the object fall upon the mirror, and are reflected by it through an 
opening in the screen or wall, forming an image in the air. Show- 
men have availed themselves of this property of concave mirrors, 
in producing the appearance of apparitions, which have terrified 
the young and the ignorant. These images have been presented 
with great distinctness and beauty, by raising a fine transparent 
cloud of blue smoke, by means of a chafing-dish, around the focus 
of a large concave mirror. 

The true focus of a concave mirror is a point equally distant from 
the centre and the surface of the sphere, of which the mirror is a 
portion. 

350. When an object is further from a concave mir- 
ror than its focus, the image will be inverted ; but when 
the object is between the mirror and its focus, the im- 
age will be upright, and grow larger in proportion as 
the object is placed nearer to the mirror. 

* A concave mirror will present a magnified, a diminished, or an equal image, 
according as the object is placed, nearer or more remote from the surface of the 
mirror ? 

The reason why convex mirrors diminish, and concave mirrors, under certain 
circumstances, magnify the images of objects, may be more clearly understood by 
the following explanation : 

According to the principle stated in No. 340, page 125, the apparent size of all 
objects depends upon the angle of vision, or the angle under which they are seen. 
According, also, to the principle stated in No. 345, page 12"/, the angle of incidence 
must be equal to the angle of reflection. [These two principles must be clearly 
understood, as the whole explanation depends upon them.] 

Do concave mirrors always present magnified imnges? What peculiar property 
belongs to concave mirrors? How can this be done ? Where is the true focus of 
a concave mirror ? 350. How does an object appear when placed farther from a 
concave mirror than its focus ? How must an object be placed to appear upright? 
In what proportion does the rise of the object increase? 

12* 



130 NATURAL PHILOSOPHY. 

351. The following facts result from the operation of 
the law already stated as the fundamental law of ca- 
toptrics ; namely, that the angles of incidence and re- 
flection are always equal. The truth of these state- 
ments may be illustrated by simple drawings ; always 
recollecting, in drawing the figures, to make the angles 
of incidence and reflection equal. The whole may al- 
so be shown by the simple experiment of placing the 
flame of a candle in various positions, before both con- 
vex and concave mirrors : 

First. With regard to Convex Mirrors. 

1. Parallel rays, reflected from a convex surface, are made to di- 
verge. 

2. Diverging ra)'s, reflected from a convex surface, are made 
more diverging. 

3. When converging rays tend towards the focus of parallel rays 
they will become parallel when reflected from a convex surface. 

4. When converging rays tend to a point nearer the surface than 
the focus, they will converge less when reflected from a convex sur- 
face. 

Now, in Fig. 102, which represents an object reflected by a convex mirror, the 
ray A A, passing from the upper extremity of the vase A B, must be reflected in 
such a manner as to make the angle of incidence A hf equal to the angle of reflec- 
tion / A E ; and, in like manner, the ray B i, proceeding from the lower extremity, 
Fig 102. must be reflected 

b ' in such a manner 

K, as to make the an- 

• ■ _^"$L gle B i g and j/ i E 

.,^,/g^t equal ; the dotted 

^. ^^^ V^j lines, /A and gt, in 

ta *>r \/ both cases repre- 

f^v^ B ^y^ 1^5 senting the perpen- 

«L ^"*^"*\ Bk/ s^^ — "■"■""^IJk diculars to the 

<^^->^_ ^^-gESs^.---, ■ ^-* > " JZ reflecting surface. 

^"^^^tt*^^^ — -~""""^ ff Now, by continu- 

^^-^jpc^^^-^ ing the lines a A 

■ton^^ — ^fiU'*^ : ^P >afc ^ ! ^r'""-"--«-«..«. and B i until they 

* M **^~*S^***^ "" S meet in D, it will 

M ^""""^^^v. be seen that the 

"C^v^ angle of vision A 

E i, or what is the 
same thing, the an- 
jle c E d is less 
lhan the angle A 
D B ; therefore, the image which subtends the angle c E d will appear less than 
the object which subtends the angle A D B. Hence, it appears that concave mir- 
rors diminish the apparent size of an object. 

In a similar manner, it may be proved, that a concave mirror presents a magnified 

ima»e, when the object is nearer to the surface of the mirror than its principal focus. 

In°Fig. 103 the ray a A, proceeding from the upper extremity of the vase a B, 



j**^ 



351. What facts are staled with regard to convex mirrors, as resulting from 
the fundamental law of catoptrics I 1. What is said of parallel rays J 2. What 
is said of diverging rays ? 3. What is said of converging rays, when they tend to- 
wards the focus of parallel rays ? 4. What is said of converging rays, when they 
tend to a point nearer the surface than the focus ? 



OPTICS. 



131 



5. If converging rays tend to a point between the focus and the 
centre, they will diverge as from a point on the other side of the 
centre, farther from it than the point towards which they converged. 

6. If converging rays tend to a point beyond the centre, they will 
diverge as from a point on the contrary side of the centre, nearer to 
it than the points towards which they converged. 

7. If converging rays tend to the centre when reflected from a 
convex mirror, they will proceed in a direction as far from the 
centre. 

Secondly. With regard to Concave Mirrors. 

8. Parallel rays, reflected from a concave surface, are made con- 
verging. 

9. Converging rays, falling upon a concave surface are made to 
converge more. 

10. Diverging rays, falling upon a concave surface, if they di- 
verge from a focus of parallel rays, become parallel. 

11. If from a point nearer to the surface than that focus, they di- 
verge less than before reflection. 



Fig. 103. 



falling on the reflecting surface so as to form the angle of incidence A h /, will 
be reflected so as to form the angle/ h E; and the ray B i, proceeding from th* 
lower extremity, 
will be reflected 
in such a man- 
ner that the an- 
gle B if shall he- 
equal to/ iE. It 
will thus be seon 
that the angle of 
vision h E i will 
be greater than 
the angle of vis- 
ion which would 
be formed by the 
object A B when 
placed at a simi- 
lar distance from 
the eye at E; or, 
in other words, 
that the distance 
from h to i is 

greater than the distance from Ato B; and the image will occupy the space 
h i in the mirror, and will, consequently, appear as much larger than the object, 
as h i exceeds A B. 

In the explanation of figures 102 and 103, it is to be observed, that no reflected 
rays from the objects A B can be seen by the eye at E, except those which proceed 
in such a manner as to make an angle, formed by lines proceeding first from the 
object to the reflecting surface, and then from the reflecting surface to the eye, 
wherever it may 




situated. It will be seen by an inspection of the figures, that 



5. What is said of converging rays, when they tend to a point between 
the focus and the centre ? 6. What is said of converging rays, when they 
tend to a point beyond the centre ? 7. What is said of converging rays, when 
they tend to the centre ? 8. What is said with regard to parallel rays, when re- 
fleeted from a concave surface ? 9. What is said of converging rays ? 10. What 
is said of diverging rays, if they diverge from a focus of parallel rays ? 11. What, 
if from a point nearer to the surface than that focus ? 



132 NATURAL PHILOSOPHY. 

12. If from a point between that focus and the centre, they con- 
verge after reflection to some point, on the contrary side of the cen- 
tre, and farther from the centre than the point from which they di- 
verged. 

13. If from a point beyond the centre, the reflected rays will con- 
verge to a point on the contrary side, but nearer to it than the point 
from which they diverged. 

14. If from the centre, they will be reflected thither again. 

The above fourteen principles, relating to rays of light reflected 
from convex and concave surfaces, all result from the same funda- 
mental laws of catoptrics, which has already been stated several 
times; namely, that when light falls on any reflecting surface it will 
invariably be reflected in such a manner as to make the angles of 
reflection equal to the angle of incidence. 

the rays will proceed precisely as is indicated by the figures, and in no other way. 
The reason why concave mirrors do not alio ays magnify the image, will appear 
from the consideration that the angle of vision, under which the image is seen, de- 
pends upon the manner in which the incident rays fall upon the reflecting surface, 
and that, consequently, if the rays proceed from a point beyond, or even at the 
principal focus, that they will form a different angle at the surface, and that, con- 
sequently, the angle of reflection will be different. As the angle of vision depends 
upon these angles, it must vary as they vary ; and it can be shown, that if the ob- 
ject be placed beyond the principal focus, that the angle of vision, beingaltered, it 
will cause the image to appear differently. 

There are three cases to be considered with regard to the effects of concave 
mirrors : 

1. When the object is placed between the mirror and the principal focus. 

2. When it is situated between its centre of concavity and that focus. 

3. When it is more remote than the centre of concavity. 

1. In the first case, the rays of light diverging after reflection, but in a less de- 
gree than before such reflection took place, the image will be larger than the ob- 
ject, and appear at a greater or smaller distance from the surface of the minor, 
and behind it. The image in this case will be erect. 

2. When the object is between the principal focus and the centre of the mirror, 
the apparent image will be behind the object, appearing very distant when the ob- 
ject is at or just beyond the focus, and advancing towards it as it recedes towards 
the centre of concavity, where, as already stated, the imtsge and the object will 
coincide. During this retreat of the object, the image will still be erect, because 
the rays belonging to each visible point will not intersect before they reach the 
eye. But in this case, the image becomes less and less distinct, at the same time 
that/the visual angle is increasing; so that at the centre, or rather a little before, 
the image becomes confused and imperfect; owing to the small parts of the object 
subtending angles too large for distinct vision, just as happens when objects are 
viewed too near with the naked eye. 

3. In the cases just considered, the images will appear erect, but in the case 
where the object is further from the mirror than its centre of concavity, the im- 
age will be inverted ; and the more distant the object is from the centre, the less 
will be its image, and the further from the said centre, or the nearer the focos, 
and the converse ; the image and object coinciding when the latter is stationed ex- 
actly at the centre, as noticed in the preceding case. 



12. What, if from a point between that focus and the centre ? 13. If from a 
point beyond the centre ? 14. If from the centre ? From what do these four- 
teen principles, stated above, result? 



OPTICS. 133 

In estimating these angles, it must be recollected, that no line is 
perpendicular to a convex or concave mirror, which will not, when 
sufficiently prolonged, pass through the centre of the sphere of 
which the mirror* is a portion. 



SECTION XV. 

Refraction of Light — Optics continued. 

352. By the refraction of light is meant its being 
turned or bent from its course ; and this always takes 
place when it passes obliquely from one medium to 
another. 

By a mediom,t in optics, is meant any substance through which 
light can pass. Thus, air, glass, water and other fluids, are media. 

* Mirrors (or looking-glasses) may be made of polished metal, or glass, with the 
back covered with an amalgam, or mixture of mercury and tinfoil. It is the 
smooth and bright surface of the mercury that reflects the rays, the glass acting 
only as a transparent case or covering, through which the rays find an easy pas- 
Bage. Some of the rays are absorbed in their passage through the glass, because 
the purest glass is not free from imperfections. For this reason the best mirrora 
are made of fine and highly polished steel. 

Concave mirrors, by the property which they possess of causing parallel rays 
to converge to a focus, are sometimes used as burning-glasses. M. Dufay made a 
concave mirror of planter of Paris, gilt and burnished, 20 inches in diameter ; with 
which he set fire to tinder, at the distance of fifty feet. But the most remarkable 
thing of the kind, on record, is the compound mirror, constructed by BufFon. He 
arranged one hundred and sixty-eight small plane mirrors in such a manner as to 
reflect radiant light and heat to the same focus, like one large concave mirror. 
With this apparatus he was able to set wood on fire at the distance of 209 feet, 
to melt lead at 100 feet, and silver at 50 feet. 

t The plural number of this word is media, although mediums is sometimes used. 
A medium is called dense or rare, in optics, according to its refractive power, and 
not according to its specific gravity. Thus, alcohol, and many of the essential 
oils, although of less specific gravity than water, have a greater refracting power, 
and are, therefore, called denser media than water. In the following list, the va- 
rious substances are enumerated in the order of their refractive power, or, in oth- 
er words, in the order of their density, the last mentioned being the densest, and 
the first the rarest •, namely, air, ether, ice, water, alcohol, alum, olive oil, oil of 
turpentine, amber, quartz, glass, melted sulphur, diamond. 

How can you prove whether aline be perpendicular to a convex or a concave 
mirror ? What is said with regard to mirrors in the note ! Of what are the best 
mirrors made ? For what are concave mirrors sometimes used ? 352. What is 
meant by the refraction of light I When does this take place? What is a me- 
dium in optics ? Give some examples of media .' In what proportion is a medium 
dense or rare I 



134 NATURAL PHILOSOPHY. 

353. There are three fundamental laws of dioptrics, 
on which all its phenomena depend, namely : 

First. When light passes from one medium to another, in a per- 
pendicular direction, it passes on in a straight line without altering 
its course. 

Second. When light passes in an oblique direction, from a rarer 
to a denser medium, it will be turned from its course and proceed 
through the denser medium less obliquely, and in a line nearer to 
a perpendicular to its surface. 

Third. When light passes from a denser to a rarer medium, 
it passes through the rarer medium in a more oblique direction, 
and in a line further from a perpendicular to the surface of the 
denser medium. 

Illustration. In Fig. 104 the line A B represents a ray of light 
passiug from air into water, in a perpendicular direction. Accord- 
ing to the first law, stated above, it will continue on in the same 
line through the denser medium to E. If 
the ray were to pass upward through the 
denser medium, the water, in the same 
perpendicular direction to the air, by the 
same law it would also continue on in 
the same straight line to A. 

But if the ray proceed from a rarer to 
a denser medium, in an oblique direction, 
as from C to B, when it enters the den- 
ser medium it will not continue on in the 
E F D same straight line to D, but, by the sec- 
ond law, staled above, it will be refracted 
or bent out of its course, and proceed in a less oblique direction to 
F, which is nearer the perpendicular ABE than D is. 

Again, if the ray proceed from the denser medium, the water, 
to the rare medium, the air, namely, from F to B ; instead of pursu- 
ing its straight course to G, it will be refracted, by the third law, 
above staled, and proceed in a more oblique direction to C, which is 
further from the perpendicular ABE than G is. 

The refraction is more or less in all cases in proportion as the rays 
fall more or less obliquely on the refracting surface. 

From what has now been stated, with regard to refraction, it will 
be seen that many interesting facts may be explained. Thus, an 
oar or a stick, when partly immersed in water, appears bent, be- 
cause we see one part in one medium, and the other in another me- 
dium ; the part which is in the water appears higher than it really 
is, on account of the refraction of the denser medium. 



353. What are the three fundamental laws of dioptrics? First? Second? 
Third? Illustrate the first rule by the line at B, in fig. 104. Illustrate the sec- 
ond rule by the line C B. Illustrate the third rule by the line F B. In what pro- 
portion does the refraction increase and diminish >. Why does an oar or a stick, 
when partly immersed in water, appear bent > Why does the part which is in the 
water appear higher than it really is ? 




OPTICS. 135 

For the same reason, when we look obliquely upon a body of wa- 
ter it appears more shallow than it really is. Bat when we look 
perpendicularly downwards, from a boat, we are liable to no such 
deception, because there will be no refraction. 

Let a piece of money be put into a cup or a bowl, and the cup and 
the eye be placed in such a position that the side of the cup will 
just hide the money from the sight, then keeping the eye still, let the 
cup be filled with water — the money will become distinctly visible. 

354. The refraction of light prevents our seeing the 
heavenly bodies in their real situation.* 

The light which they send to us is refracted in passing through 
the atmosphere, and we see the sun, the stars, &c. in the direction 
of the refracted ray. In consequence of this atmospheric refrac- 
tion the sun sheds his light upon us earlier in the morning and later 
in the evening, than we should otherwise perceive it. And when 
the sun is actually below the horizon, those rays which would oth- 
erwise be dissipated through space, are refracted by the atmosphere 
towards the surface of the earth, causing twilight. The greater 
the density of the air the higher is its refractive power, and, conse- 
quently, the longer the duration of twilight. 

355. When a ray of light passes from one medium 
to another, and through that into the first again, the 
two refractions being equal, and in opposite directions, 
no sensible effect is produced. 

This explains the reason why the refractive power of flat window 
glass produces no effect on objects seen through it. The rays suffer 
two refractions, which, being in contrary directions, produce the 
same effect as if no refraction had taken place. 

* There is another reason, also, why we do not see the heavenly bodies in their 
true situation. Light, though it moves with great velocity, is about 8 1 2 minutes 
iu its passage from the sun to the earth, so that when the rays reach us, the sun 
has quitted the spot he occupied on their departure ; yet we see him in the direc- 
tion of those rays, and, consequently, in a situation which he abandoned eight 
minutes and a half before. The refraction of light does not affect the appearance 
of the heavenly bodies when they are vertical, that is, directly over our heads, 
because the rays then pass perpendicularly, a direction incompatible with refrac- 
tion. 

It may here also be remarked that it. is entirely owing to the refraction of the 
atmosphere that the heavens appear bright in the day time. If the atmosphere 
had no refractive power, only that part would be luminous in which the sun is 
placed ; and on turning our back to the sun, the whole heavens would appear as 
dark as in the night ; we should have no twilight, but a sudden transition from 
the brightest sunshine to darkness, immediately upon the setting of the sun. 

Why does a body of water, when viewed obliquely, appear more shallow than 
it really is? In what direction can we look so as to cause no refraction? What 
experiment is here related >. 354. Why do we not see the heavenly bodies in their 
real situation ? In what direction do we see them? What causes twilight ? Upon 
what does the duration of twilight depend ? What other reason is given, in the 
note, why we do not see the heavenly bodies in their true situation? When does 
the refraction of light not affect the appearance of the heavenly bodies! Why 
do the heavens appear bright in the day time! 355. What, effect is produced 
when a ray of light passes from one medium to another, and through that into the 
first again ? Why ? Why docs the refractive power of flat window-glass pro- 
duce no effect on objects seen through it ? 



136 



NATURAL PHILOSOPHY. 




356. A lens is a glass, which, according to its pecu- 
liar form, causes the rays of light to converge to a fo- 
cus, or disperses them further apart, according to the 
laws of refraction. 

357. There are various kinds of lenses, named ac- 
cording to their focus ; but they are all to be consider- 
ed as portions of the internal or external surface of a 
sphere. 

Fig. 105. 

C E D A sin S le con - 

vex lens has one 
side flat and the 
other convex ; as 
G A in Fig. 105. 
A single con- 
cave lens is flat on 
one side and con- 
cave on the other, 
as B in Fig. 105. 
A double con- vex lens is convex on both sides, as C, Fig. 105. 
A double concave lens is concave on both sides, as D, Fig. 105. 
A meniscus is convex on one side and concave on the other, as 
E, Fig. 105. 

The axis of a lens is a line passing through the centre ; thus, 
F G, Fig. 105, is the axis of all the five lenses. 

358. The peculiar form of the various kinds of len- 
ses causes the light which passes through them to be 
refracted from its course. (According to the laws stated 
in No. 353.) 

It will be remembered that, according to the laws stated in No. 
353, light, in passing from a rarer to a denser medium, is refracted 
towards the perpendicular ; and, on the contrary, that in passing 
from a denser to a rarer medium, that it is refracted further from 
the perpendicular. In order to estimate the effect of a lens, we 
must consider the situation of the perpendicular, with respect to 
the surface of the lens. Now, a perpendicular, to any convex or 
concave surface, must always, when prolonged, pass through the 
centre of sphericity ; that is, in a lens, the centre of the circle of 

356. What is a lens ? 357. How are all lenses to be considered ? What is a 
■ingle convex lens? What part of fig. 105 represents a single convex lens? 
What is a single concave lens.' What part of fig. 105 represents a single concave 
lens ? What is a double convex lens ? What part of fig. 105 represents a double 
convex lens ? What is a double concave lens >. What part of fig. 105 represents 
a double concave lens I What is a meniscus ? What part of fig. 105 represents a 
meniscus ? What is the axis of a lens ? What line, in fig. 105 represents the 
axis of all the five lenses > 358. What is stated in No. 358 with regard to the 
form of the lenses ? How is light refracted in passing from a rarer to a denser 
medium ? How, in passing from a denser to a rarer ? What must be considered 
in estimating the effect of lenses? Through what must a perpendicular, to any 
convex or concave surface, always, when prolonged, pass I 



OPTICS. 137 

Which the lens is a portion. By an attentive observation, therefore, 
of the Jaws above stated, and of the situation of the perpendicular 
on each side of the lens, it will be found in general, — 

First, That convex lenses collect the rays into a focus, and, con- 
sequently, magnify objects at a certain distance. 

Second, That concave lenses disperse the rays, and, consequent- 
ly, diminish objects seen through them. 

359. The focal distance of a lens is the distance from 
the middle of the glass to the focus. This, in a single 
convex lens, is equal to the diameter of the sphere of 
which the lens is a portion ; and in a double convex 
lens is equal to the radius of a sphere of which the 
lens is a portion. 

360. When parallel rays* fall on a convex lens, that 
only which falls in the direction of the axis of the lens 
is perpendicular to its surface, and will continue on in 
a straight line through the lens. The other rays, fall- 
ing obliquely, are refracted to the axis and will meet in 
a focus. 

It is this property of a convex lens which gives it its power as a 
burning glass. All the parallel rays of the sun which pass through 
the glass, are collected together in the focus ; and, consequently, 
the heat at tl A e focus is to the common heat of the stm, as the area of 
the glass is to the area of the focus. Thus, if a lens, four inches 
in diameter, collect the sun's rays into a focus, at the distance of 
twelve inches, the image will not be more than one tenth of an inch 
in diameter; the surface of this little circle is 1600 times less than 
the surface of the lens, and, consequently, the heat will be 1600 
times greater at the focus than at the lens, t 

* The rays of the sun are considered parallel at the surface of the earth. 

t The following effects were produced by a large lens, or burning glass, two feet 
in diameter, made at Leipsic, in 1691. Pieces of lead and tin were instantly melt- 
ed ; a plate of iron was soon rendered red hot, and afterwards fused, or molted, 
and a burnt brick was converted into yellow glass. A double convex lens, three 
feet in diameter, and weighing 212 pounds, made by Mr. Parker, in England, melt- 
ed the most refractory substances. Cornelian was fused in 75 seconds, a crystal 
pebble in 6 seconds, and a piece of white agate in 30 seconds. This lens was pre- 
sented by the King of England to the Emperor of China. 

What is stated with regard to convex lenses '. What, with regard to concave 
lenses ? 359. What is the focal distance of a lens > To what is this equal in a 
single convex lens I To what is it equal in a double convex lens ? 360. When 
parallel rays fall on a convex lens, which one is perpendicular to its surface; How 
are the other rays, falling obliquely, refracted ? What property of a convex lens, 
gives it its power as a burning glass ? Where are all the parallel rays of the sun, 
which pass through the glass, collected ? How does the heat at the focus compare 
with the common heat of the sun ? What is related in the note with regard to 
the effects of lenses produced by burning glasses .' 

13 



138 NATURAL PHILOSOPHY. 

361. The following effects result from the laws of 
refraction, stated in No. 353 ; and, first, with regard to 
convex surfaces. 

1. Parallel rays passing out of a rarer into a denser medium, 
through a convex surface, will become converging. 

2. Diverging rays will be made to diverge less, to become paral- 
lel, or to converge, according to the degree of divergency before re- 
fraction, or of the convexity of the surface. 

3. Converging rays, towards the centre of convexity, will suffer 
no refraction. 

4. Rays converging to a point beyond the centre of convexity, 
will be made more converging. 

5. Converging rays towards a point nearer the surface than the 
centre of convexity, will be made less converging by refraction. 

[ When the rays proceed out of a denser into a rarer medium, the 
reverse occurs in each case.] 

Secondly. With regard to Concave Surfaces. 

6. Parallel rays, proceeding out of a rarer into a denser medium, 
through a concave surface, are made to diverge. 

7. Diverging rays are made to diverge more— to suffer no refrac- 
tion — or to diverge less, according as they proceed from a point be- 
yond the centre, from the centre, or between the centre and the sur- 
face. 

8. Converging rays are made less converging, parallel, or diverg- 
ing, according to their degree of convergency before refraction.* 

[ When the rays proceed out of a denser into a rarer medium, the re- 
verse takes place in each case.] 

362. Double convex, and double concave glasses, or 
lenses, are used in spectacles, to remedy the defects of 
the eye, when by age it becomes too flat, or loses a 
portion of its roundness; or when by any other cause 
it assumes too round a form, as in the case of short 
sighted, (or, as they are sometimes called, nearsighted) 

* The above eight principles are all the necessary consequence of the operation 
of the three laws mentioned in number 353. The reason that so many different 
principles are produced, by the operation of those laws, is, that the perpendiculars 
to a convex or concave surface are constantly varying, so that no two are parallel. 
But in flat surfaces the perpendiculars are parallel ; and one invariable result is 
produced by the rays when passing from a rarer to a denser, or from a denser to a 
rarer medium, having a flat surface. 

361. 1. What is the first effect related as resulting from the laws of refraction, 
stated in No. 345, with regard t.o convex surfaces? 2. What is said of diverging 
rays; 3. What is said of converging rays towards the centre of convexity .' 4. 
What of rays converging to a point beyond the centre of convexity ? 5. What of 
rays converging to a point nearer the surface than the centre of convexity ! When 
the rays proceed out of a denser into a rarer medium, what occurs; 6. What is 
stated, in No. 6, with regard to concave surfaces ! 7. What is said of diverging 
rays? 8. What is said of converging rays .» Of what are the above eight prin- 
ciples the necessary consequence ? What is the reason that so many different 
principles are produced by the operation of these laws ! 3G2. For what are double 
convex and concave glasses, or lenses, used in spectacles .' 



OPTICS. 



139 



ous Humor. 7 
The Sclerotica. 

Fig. 106. 




persons. Convex glasses are used when the eye is too 
flat, and concave glasses when it is too round.* 

363. The eye is composed of a number of coats, or 
coverings, within which are enclosed a lens, and cer- 
tain humors, in the shape, and performing the office of 
convex lenses. 

364. The different parts of the eye are : 1. The 
Cornea. 2. The Iris. 3. The Pupil. 4. The Aque- 
ous Humor 5. The Crvstalline Lens. 6. The Vitre- 

The Retina. 8. The Choroid. 9. 
10. The Optic Nerve. 

Illustration. Fig. 106 represents 
a front view of the eye, in which 
a a represents the cornea, or, as it 
is commonly called, the white of 
the eye ; e e is the Iris, which is of 
different colors in different per- 
sons ; and we call a person's eye 
black, blue, &c. according to the 
color of the Iris. The Iris has a 
circular opening in the centre, 
called the pupil, p, which contracts 
in a strong light, and expands m a 
faint light, and thus regulates the 
quantity which is admitted to the tender parts in the interior of the eye. 

Fig. 107 represents aside 
view of the eye, laid open, 
in which b b represents the 
cornea, e e the Iris, d d the 
pupil,// the aqueous hu- 
mor, g g the crystalline 
lens, hh the vitreous humor, 
i i i i i the Retina, c c the 
choroid a a a a a the scle- 
rotica, and n the optic 
nerve. 

The cornea forms the an- 
terior portion of the eye. 
It is set in the sclerotica in 
the same manner as a erys- 

* These lenses or glasses are generally numbered by opticians, according to their 
degree of convexity or concavity ; so that by knowing the number that fits the 
eye, the purchaser can generally 'be accommodated without the trouble of trying 
many glasses 

What glasses are used when the eve is too flat I What are used when the eye 
is too round ? 363. Of what is the eye composed ? 264. What are the different 
parts of the eye ? First? Second?' Third > Fourth? Fifth I Sixth; Seventh 2 
Eighth ? Ninth .' Tenth > What does fig. 106 represent ? Explain the fig. What 
does fig. 107 represent ? Explain the fig. What part of the eye does the cornea 
form ? 




!40 NATURAL PHILOSOPHY. 

tal of a watch is set in the case. Its degree of convexity varies in 
different individuals and in different periods of life. As it covers 
the pupil and the iris, it protects thern from injury. Its principal 
office is to cause the light which reaches the eye, to converge to the 
axis. Part of the light, however, is reflected by its finely polished 
surface, and causes the brilliancy of the eye. 

The Iris is so named from its being of different colors. It is a 
kind of circular curtain, placed in the front of the eye to regulate 
the quantity of light passing to the back part of the eye. It has a 
circular opening in the centre, which it involuntarily enlarges or 
diminishes. 

The pupil is merely the opening in the iris, through which the 
light passes to the lens behind. It is always circular in the human 
eye, but in quadrupeds it is of different shape. When the pupil is 
expanded to its utmost extent it is capable of admitting ten times the 
quantity of light that it does when most contracted.* In cats and 
other animals, which are said to see in the dark, the power of dila- 
tation and contraction is much greater; it is computed that their 
pupils may receive one hundred times more light at one time than 
at another. The light only, which passes the pupil, can be of use 
in vision ; that which falls on the iris is reflected, returns through 
the cornea, and exhibits the color of the iris. 

The aqueous humor is a watery fluid,t as clear as the purest wa- 
ter. In shape it resembles a meniscus, and, being situated between 
the cornea and the crystalline lens, it assists in collecting and trans- 
mitting the rays of light from external objects to that lens. 

The crystalline lens is a transparent body, in the form of a 
double convex lens, placed between the aqueous and vitreous hu- 
mors. Its office is not only to collect the rays to a focus, on the reti- 
na, but also to increase the intensity of the light which is directed 
to the back part of the eye. 

The vitreous humor (so called from its resemblance to melted 
glass,) is a perfectly transparent mass, occupying the globe of the 
eye. Its shape is like a meniscus, whose convexity greatly exceeds 
the concavity. 

* When a person comes from a dark place into a strong light, the eyes suffer 
pain, because, the pupil being expanded, admits a larger quantity of light to rush 
in, before it has had time to contract. And when we go from a strong light into 
a faint one, we at. first imagine ourselves in darkness, because the pupil is then 
Contracted, and does not instantly expand. 

t The author is aware of the tautology in this expression, but as he is writing 
for the instruction of children, and unlettered persons, ho deems it necessary to 
make his explanations as simple as possible. 

Is its degree of convexity the same in all persons and all periods of life >. What 
is its principal office >. From what does the iris take its name ? What is the use 
of the iris; What is the pupil? What is its form in the human eye ! How 
much more light is the pupil capable of admitting, when expanded to its utmost 
extent, than when most contracted i What is said of those animals which are 
said to see in the dark? What light, only, is of use in vision ! What becomes 
of the light which falls on the iris? What is the aqueous humor » What is its 
form? Of what use is it! What is the crystalline lens? What is its officer 
What is the vitreous humor ? Why do persons sometimes experience pain when 
passing from a dark place into strong light > What is the shape of the vitreous 




OPTICS. 141 

Fi ?- 103 - In Figure 108 the shape of 

the aqueous and vitreous hu- 
mors, and the crystalline lens is 
presented, a is the aqueous hu- 
mor, which is a meniscus, b the 
crystalline lens, which is a double 
'Zfrf / k ) n convex lens, and c the vitreous 

humor, which isj also, a menis- 
cus, whose concavity has a 
smaller radius than its convexity. 
The retina is the seat of vi- 
sion or sight. The rays of light 
being refracted in their passage 
by the other parts of the eye, are brought to a focus in the retina, 
where an inverted image of the object is represented. 

The choroid is the inner coat or covering of the eye. Its outer 
and inner surface is covered with a substance called the pigmentum 
nigrum, (or black paint.) Its office is, apparently, to absorb the rays 
of light immediately after they hare fallen on the retina. It is the 
opinion of some philosophers that it is the choroid and not the reti- 
na, which conveys the sensation produced by rays of light to the 
brain. 

The sclerotica is the outer coat of the eye. It derives its name 
from its hardness. Its office is to preserve the globular figure of the 
eye. and defend its more delicate internal structure. To the sclero- 
tica are attached the muscles which move the eye. It receives the 
cornea, which is inserted in it somewhat like a watch glass in its 
case. It is pierced by the optic nerve, which, passing through it, 
expands over the inner surface of the choroid, and thus forms the 
retina. 

The optic nerve is the organ which carries the impressions made 
by the rays of light, (whether by the medium of the retina or the 
choroid,) to the brain, and thus produces the sensation of sight * 

365. The eye is a natural camera obscura, and the 
images of all objects seen by the eye are represented 
on the retina, in the same manner as the forms of ex- 
ternal objects are delineated in that instrument. (See 
No. 339, note.) 

* For the above description of the eye and its parts, the author is mainly indebt- 
ed to Paxton's Introduction to the Study of Anatomy, edited by Dr. Lewis of this 
citv. 



Explain fig. 108. "What is tbe retina ! What is the choroid ? By what is its 
outer and inner surface covered ? What is its office ! Whit is the opinion of 
some philosophers with regard to the choroid: What is the sclerotica? From 
what does it derive- its name ! What is its office ? What are attached to the 
sclerotica >. What is the optic nerve ! 365. What is stated, in No. 365, with re- 
gard to the representations on the retina of the images of all objects seen by the 
eye? 

13* 



142 NATURAL PHILOSOPHY. 

Fig. 109 represents only those parts of the eye which are most 

essential. The image is formed thus. The rays from the object c d, 

diverging toward the eye. enter the cornea c, and cross one another 

in their passage, through the crystalline lens d, by which they are 

Fig. 109. 




made to converge on the retina, where they form the inverted* im- 
age, fe. 

366. The convexity of the crystalline humor is in- 
creased or diminished hy means of two muscles to 
which it is attached. By this means the focus of the 

* Although the image i9 inverted on the retina, we see objects erect, because all 
the images, formed on the retina have the same relative position which the ob- 
jects themselves have ; and as the rays all cross each other, the eye is directed up- 
wards, ts receive the rnys which proceed from 'he upper part of an object, and 
downwards, to receive those which proceed from the lower part. 

A distinct image is also formed on the retina of each eye; but as the optic nerves 
of the two eyes unite, or cross each other before they reach the brain, the impres- 
sions received by the two nerves are united, so that only one idea is excited, and 
objects are seen single. Although an object may be distinctly seen with r-nly 
one eye, it has been calculated that the use of both eyes makes a difference of about 
one twelfth. 

From the description now given of the eye, it may be seen what are the defects 
which are remedied by the use of concave and convex lenses ; and how the use of 
these lenses remedies them. When the crystalline humor of the eye is too round, 
the rays of light, which enter the eye, are converged to a focus before they reach 
the retina, and, therefore, the imago will not be distinct ; and when the crystal- 
line humor is too flat, (as is often the case with old persons,) the rays will not be 
converged on the retina, but tend to a point beyond it. A convex glass, by assist- 
ing the convergency of the crystalline lens, brings the rays to a focus on the retina, 
and produces distinct vision. 

The eye is also subject to imperfection by reason of the humors losing their 
transparency, either by age or disease. For these imperfections no glasses offer a 
remedy without the aid of surgical skill. The operation of couching and remov- 
ing cataracts' from the eye consists in making a puncture or incision through 
which the diseased part may escape. Its office is then supplied hy a lens. If, 
however, the operator, by accident or want of skill, permits the vitreous hu- 
mor to escape, the globe of the eye immediately diminishes in size, and total 
blindness is the inevitable result. 

Explain fig. 109. Why do the objects appear erect when the images are invert- 
ed ? Why do we see only one image, when an image is formed on both eyes? 
What are the defects which are remedied by the use of concave and convex len- 
ses ? In what other way is the eye subject to imperfection ? Is there any remedy 
for this ? 366. By what is the convexity of the crystalline humor increased or 
diminished I 



OPTICS. 143 

rays which pass through it, constantly falls on the reti- 
na ; and an equally distinct image is formed, both of dis- 
tant objects and those which are near. 

367. A single microscope consists simply of a con- 
vex lens, commonly called a magnifying glass ; in the 
focus of which the object is placed, and through which 
it is viewed. 

By means of a microscope the rays of light from an object are 
caused to diverge less ; so that when they enter the pupil of the 
eye they fall parallel on the crystalline lens, by which they are re- 
fracted to a focus on the retina. 

Fig. 110 represents a convex lens, or single microscope, C P. 
The diverging rays from the object A B are refracted in their pas- 
sage through the lens C P, (See 2 under Ao. 361,) and made to 
Fig. no. 




fall parallel on the crystalline lens, by which they are refracted to 
a focus on the retina R R ; and the image is thus magnified, because 
the divergent rays are collected by the lens and carried to the retina. 
Those lenses or microscopes which have the shortest focus, have 
the greatest magnifying power ; and those which are the most bulg- 
ing or convex, have the shortest focus. But as the protuberance of 
a large lens prevents the eye from approaching very near to 
an object, this inconvenience is remedied by making the lens ex- 
ceedingly small. It may then be spherical without occupying much 
space, and thus unite the advantages of a short focus, and of allow- 
ing the eye to approach very near to an object. 

368. A double microscope consists of two convex 
lenses, by one of which a magnified image is formed, 
and by the other this image is carried to the retina of 
the eye. 

What is effected by this means ? 367. What is a single microscope ? What 
is the use of this microscope! What fig. represents a microscope; Explain the 
figure. What lenses have the greatest magnifying power? What lenses have the 
shortest focus ; Of what does a double microscope consist .' What is the use of 
these two lenses ! 



144 



NATURAL PHILOSOPHY. 



Fig. Ill represents the effect produced by the lenses of a double 
microscope. The rays which diverge from the object A B are 
collected by the lens)L M, (called the object glass, because it is near- 
est to the object,) and form an inverted magnified image at C D. 

Fig. ill. 




The rays which diverge from this image are collected by the lens, 
N O, (called the eye glass, because it is nearest to the eye.) which 
acts on the principle of the single microscope, and forms a still 
more magnified image on the retina R R. 

369. The solar microscope, is a microscope with a 
mirror attached to it, upon a moveable joint, which can 
be so adjusted as to receive the sun's rays and reflect 
them upon the object. It consists of a tube, a mirror 
or looking glass, and two convex lenses. The sun's 
rays are reflected by the mirror through the tube upon 
the object ; the image of which is thrown upon a white 
screen, placed at a distance to receive it. 

The microscope, as above described, is used for viewing transpa- 
rent objects only. When opaque objects are to be viewed, a mirror 
is used to reflect the light on the side of the object ; the ima?e is 
then formed by light reflected from the object, instead of being trans- 
mitted through it. 

The magnifying power of a single microscope is ascertained by 
dividing seven inches, the least distance at which an object can be 
seen by the naked eye, by the focal distance of the lens. Thus, if 
the focal distance of a lens be only the 1-4 of an inch, then the di- 
ameter of an object will be magnified 28 times, (because 7, divided 
by 1-4, is the satne as multiplying 7 by 4,) and the surface will be 
magnified 784 times. 



What does fig. Ill represent ? Explain the figure. 369. What is the solar mi- 
croscope > Of what does it consist '. By what, in this microscope, are the sun'a 
rays reflected, and upon what >. For viewing what ohjects, only, is the microscope, 
above described, used? How do those microscopes used for viewing opaque ob- 
jects, differ from these ? How is the image then formed ? How is the magnifying 
power of a single microscope ascertained ? Illustrate this. 



OPTICS. 



145 



The magnifying power of the compound microscope is found in a 
similar manner, by ascertaining the magnifying power first of one 
lens, and then of the other. 

The magnifying power of the solar microscope is in proportion as 
the distance of the image, from the object glass, is greater than that 
of the object itself from it. Thus, if the distance of the object from 
the object glass be 1-4 of an inch, and the distance of the image, or 
picture, on the screen, be ten feet, or 120 inches, the object will be 
magnified in length 480 times, or, in surface, 230,000 times.* 

370. The magic lantern is an instrument constructed 
on the principle of the solar microscope, with this dif- 
ference, that the light is supplied by a lamp instead 
of the sun. 

The objects to be viewed by the magic lantern are generally paint- 
ed with transparent colors, on glass slides, which are received into 
an opening in the front of the lantern. The light from the lamp, 
in the lantern, passes through them, and carries the pictures, paint- 
ed on the slides, through the lenses, by means of which a magnified 
image is thrown upon the wall, on a white surface prepared to re- 
ceive it. 

Fig. 112 represents the magic lantern. The rays of light from 
the lamp are received upon the concave mirror e, and reflected to 
Fig. 112. 




the convex lens c, which is called the condensing lens, because it 
concentrates a large quantity of light upon the object painted on the 

* A lens may be caused to magnify or to diminish an object. If the object be 
placed at a distance from the focus of a Jens, and the image be formed in or near 
the focus, the image will be diminished ; but if the object be placed near the fo- 
cus, the image will be magnified. 



How is the magnifying power of the compound microscope ascertained? In 
what proportion is the magnifying power of the solar microscope? Illustrate 
this. How may a lens be made to magnify or diminish an object ? 370. What is 
the magic lantern? What figure represents a magic lantern .' Explain the figure. 
In what proportion will the size of the image increase or diminish ? 



146 NATURAL PHILOSOPHY. 

slide, inserted at b. The rays from the illuminated object at b are 
carried divergent through the lens a, forming an image on the screen 
at/. The image will increase or diminish in size, in proportion to 
the distance of the screen from the lens a. 

371. A telescope is an instrument for viewing distant 
objects. There are two kinds of telescopes, namely, 
the refracting telescope and the reflecting telescope. 
A refracting telescope is one in which the object itself 
is viewed, through the medium of a number of lenses. 
A reflecting telescope is one in which the image of the 
object is reflected from a concave mirror, within the 
tube of the telescope, and viewed through a number of 
lenses.* 

There are two kinds of refracting telescopes, called the astronom- 
ical telescope, or night glass, and the terrestial telescope, or day 
glass.t In the former, or night glass, there are but two lenses or 
glasses, but the object is viewed in an inverted position. As the 
glass is used principally for viewing the heavenly bodies, the inver- 
sion of the image produces no inconvenience. In the latter, or day 
glass, two additional lenses are introduced to give the image its nat- 
ural position. 

Fig. 113 represents a night glass, or astronomical telescope. It 

consists of a tube, A B C D, containing two glasses, or lenses. 

The lens A B, having a longer focus, forms the object glass; the 

other lens, D C, is the eye glass. The rays from a very distant body, 

Fig. 113. 




as a star, and which may be considered parallel to each other, are re- 
fracted by the object glass A B to a focus at K. The image is then 
seen through the eye glass D C magnified as many times as the fo- 
cal length of the eye glass is contained in the focal length of the 

* The image of the object seen through a refracting telescope is never so clear 
and perfect as that obtained by the reflecting telescope ; because the dispersion of 
colors which every lens produces, in a greater or 1'ss degree, renders the image dull 
and indistinct, in proportion to the number of lenses employed. 

t Some glasses or teleseopes are marked " Night and Day." These have four 
glasses, two of which can be removed when the heavenly bodies are viewed. 

371. What is a telescope? How many kinds of telescopes are there? What 
are they ? What is a refracting telescope ? What is a reflecting telescope? Why 
is the image of an object, seen through a refracting telescope, less clear and per- 
fect than when seen through a reflecting telescope ? How many kinds of refract- 
ing telescopes are there? What are they? How do they differ, the one from the 
other? What does figure 113 represent >. Explain the figure. 



OPTICS. 



147 



object glass. Thus, if the focal length of the eye glass, D C be 
contained 100 times in that of the object glass, A B, the star will be 
seen magnified 100 times. It will be seen by the figure that the im- 
age is inverted; for the ray M A, after refraction, will be seen in 
the direction C O, and the ray N B, in the direction D P. 

Fig. 114 represents a day glass or terrestrial telescope, common- 
ly called a spy glass. This, likewise, consists of a tube, ABHG, 
containing four lenses, or glasses, namely, A B, C D, E F and G 
H. The lens A B is the object glass, and G H the eye glass. The 
two additional eye glasses, E F and C D 3 are of the same size and 
Fig. 114. 




shape, and placed at equal distances from each other, in such a man- 
ner that the focus of the one meets that of the next lens. These 
two eye glasses, E F and C D, are introduced for the purpose of col- 
lecting the rays proceeding from the inverted image M N into a 
new upright image, between G H and E F, and the fmas;e is then 
seen through the last eye glass G H uuder the angle of vision, P O Q.. 
Fig. 115 represents a reflecting telescope. This consists also of 
a large tube, containing two concave metallic mirrors, {See number 
349,) A B and C, with two plano-convex eye glasses. The mir- 

A Fig. 115. 




rors are placed at a little more than the sum of their focal distance 
from each other. The parallel rays r r, coming from a distant ob- 
ject, are reflected to a focus g by the concave mirror, A B, and thus 
form an inverted image at g ; the diverging rays proceeding from 
this image are again reflected by the small mirror C, and received 
by the eye glass F, through a hole in the middle of the mirror A B. 
The eye glass F collects these reflected rays into a new image, at I 
and this image is seen magnified through the second eye glass, G. 



What does fig. 114 represent ? Explain the figure. What does fig. 115 repre- 
sent? Explain the figure. 



148 NATURAL PHILOSOPHY. 

In reflecting telescopes, mirrors are used to bring the image near 
the eye, and a lens or eye glass to magnify the image. 

The advantage of the reflecting telescope, is, that mirrors, whose 
focus is six feet, will magnify as much as lenses whose locus is a 
hundred feet. 

372. That part of the science of optics which relates 
to colors is called Chromatics. 

Colors do not exist in the bodies themselves, but are caused by 
the peculiar manner in which the light is reflected from their surfa- 
ces. [See No. 21.] 

373. Light is composed of rays of different colors, 
which may be separated by a prism.* 

374. A prism is a solid triangular, or three-sided 
piece of highly polished glass, generally six or eight 
inches long.f [See Fig. 116.] 

375. The colors which enter into the composition of 
light are seven, namely, red, orange, yellow, green, 
blue, indigo, and violet. Each of these have different 
degrees of refrangibility. 

376. When light is made to pass through a prism, the 
different colored rays are separated, and form an image 
on a screen or wall, in which the colors will be arrang- 
ed in the order in which they are enumerated in No. 
375. 

Illustration. Fig. 116 represents rays of light passing from the 
aperture, in a window-shutter, A B, through the prism P. Instead 
of continuing in a straight course to E, and there forming an im- 
age, they will be refracted, in their passage through the prism, and 
form an image on the screen, C D. But as the different colored 
rays have different degrees of refrangibiliry, (See No. 352,) or, in 
other words, suffer different degrees of refraction, those which are 
refracted the least will fall upon the lowest part of the screen, and 

* This discovery was mado by Sir Isaac Newton. 

f A prism may bo made of three pieces of plate glass, about six or eight inches 
long, and two or three broad, joined together at their edges, und made water tight 
by putty. The ends may be fitted to a triangular piece of wood, in one of which 
an aperture is made by which to fill it with water, and thus to give it the appear- 
ance and the refractive power of a solid prism. 

Why are mirrors used in reflecting telescopes ? What is the use of the lens ? 
What is the advantage of the reflecting telescope? 372. What is chromatics? 
What causes color ? 373. Of what is light composed? How can these rays be 
separated.' By whom was this discovery made? 374. What is a prism i How 
may a prism be mado? 375. How many colors enter into the composition of light ? 
What are they ? Do these rays all have the same degree of refrangibility ? 376. 
What takes place when light is made to pass through a prism i 



OPTICS. 



149 



those which are refracted the most will fall upon the highest part. 

The red rays, therefore, suffering the smallest degree of refraction, 

Fig. 116. 




fall on the lowest part of the screen, and the remaining colors are 
arranged in the order of their refraction* 

If the colored rays, which have been separated by a prism, fall 
upon a convex lens, they will converge to a focus, and appear 
white. Hence, it appears, that white is not a simple color, but is 
produced by the union of several colors. 

The spectrum, formed by a prism, being divided into 360 parts, it 
is found that the red occupies 45 of those parts, the orange 27, the 
yellow 48, the green 60, the blue 60, the indigo 40, and the violet 80. 
By mixing the seven primitive colors in these proportions, a white 
is obtained ; but, on account of the impurity of all colors, it will be 
of a dingy hue. If the colors were more clearly and accurately 
denned, the white, thus obtained, would appear more pure also. 
An experiment to prove what has just been said may be thus per- 

* It is supposed that the red rays are refracted the least, on account of their 
greater momentum, and that the blue, indigo, and violet are refracted the most, 
because they have the least momentum. The same reason, it is supposed, will ac- 
count for the red appearance of the sun. through a fog, or at rising and setting. 
The increased quantity of the atmosphere, which the oblique rays must traverse, 
and its being loaded with mists and vapors, which are usually formed at those 
times, prevents the other rays from reaching us. 

A similar reason w ill account f>r the blue appearance of the sky. As these rays 
have less momentum, they cannot traverse the atmosphere so readily as the other 
rays, and they are, therefore, reflected back to our eyes by the atmosphere. If the 
atmosphere did not reflect any rays the skies would appear perfectly black. 



Explain figure 116. Why do the red rays fall on the lowest part of the screen? 
What is supposed with regard to the red rays? What, with regard to the blue, 
indigo and \ioletrays? Why does the sun appear red through a fog! Why 
does the sky appear of a blue color? What would be the appearance of the sky 
if the atmosphere did not reflect any rays ? Is white a simple color? How is 
it produced ! The spectrum formed by a prism, being divided into 360 parts, how 
many of these parts does the red occupy? The orange. 1 The yellow' The green? 
The blue ? The indigo ? The violet ? 



14 



150 NATURAL PHILOSOPHY. 

formed. Take a circular piece of board, or card, and divide it in- 
to parts, by lines drawn from the centre to the circumference. Then, 
having painted the seven colors in the proportions above named, 
cause the board to revolve rapidly around a pin or wire at the cen- 
tre. The board will then appear of a white color. From this it is 
inferred that the whiteness of the sun's light arises from a due mix- 
ture of all the primary colors. 

The colors of all bodies are either the simple colors, as refracted 
by the prism, or such compound colors as arise from a mixture of 
two or more of them* 

377. The rainbow is produced by the refraction of 
the sun's rays in their passage through a shower of rain ; 
each drop of which acts as a prism in separating the 
colored rays, as they pass through it. 

This is proved by the following considerations. First, A rainbow 
is never seen except when rain is falling, and the sun shining at the 
same time ; and that the sun and the bow are always in opposite 
parts of the heavens ; and, secondly, that the same appearance may 
be produced artificially, by means of water thrown into the air, 
when the spectator is placed in a proper position, with his back to 
the sun ; and, thirdly, that a similar bow is generally produced by 
the spray which arises from large cataracts, or waterfalls, such, for 
instance, as thtTFalls of Niagara." 

378. The color of all bodies depends upon the rays 
which they reflect. 

Some bodies absorb all the rays which they receive except the red 
rays. These bodies, therefore, appear of a red color — some reflect 
the green, and absorb all the others — these will appear of a green 
color; and, in general, bodies appear of the color of those rays 
which they reflect, while they absorb all the other rays. Sometimes 
a body reflects a portion of the rays of several colors. The body 
will then appear of a compound color, composed of the various 
colors which it reflects. "When a body reflects all the rays, it appears 
wkUe — when it absorbs all the rays, it appears black. White, then, 
is a mixture of all the primitive colors, and black is the deprivation 
of all color. 

From what has now been said it appears that no body has a per- 
manent or intrinsic color of its own — but that color, as well as 
weight, are accidental, and not essential properties. (See Ao. 21, 
page 7.) All substances appear of the same color, or rather, more 
properly speaking, are deprived of all color, in the dark. Light, 
from whatever source it proceeds, is of the same nature, composed 

* From the experiments of Dr. Wollaston, it appears that the seven colors, 
formed by the prism, may be reduced to four, namely, red , green, blue and violet ; 
and that the other colors are produced by combinations of these. 

What are the colors of all bodies > What appears from the experiments of Dr. 
Wollaston >. 377. How is the rainbow produced .' How is this proved; First » Sec- 
ond? Third? 378. Upon what does the color of all bodies depend? Of what 
color do bodies generally appear! When will a body appear of a compound color ? 
Of what color will a body appearthat reflects all the rays' When will a body 
appear black I Is color an essential property of a body ? 



OPTICS. 151 

of the various colored rays; and although some substances appear 
differently by candle-light from what they appear by day, this result 
may be supposed to arise from the weakness or want of purity in 
artificial light. 

There can be no light without colors, and there can be no colors 
without light. 

That the above remarks, in relation to the colors of bodies, are 
true, may be proved by the following simple experiment. Place a 
colored body in a dark room, in a ray of light that has been refracted 
by a prism ; the body, of whatever color it naturally is, will appear 
of the color of the ray in which it is placed ; for, since it receives 
no other colored rays, it can reflect no others.* 

379. A multiplying glass is a convex lens, one side of 
which is ground down into several flat surfaces. 

When an object is viewed through a multiplying glass, it will be 
multiplied as many times as there are flat surfaces on the lens. 
Thus, if one lighted candle be viewed through a lens, having 
twelve flat surfaces, twelve candles will be seen through the lens. 
The principle of the multiplying glass is the same with that of a 
convex or concave lens. 

380. The Kaleidescopef consists of two reflecting 
surfaces, or pieces of looking-glass, inclined to each 
other at an angle of 60 degrees, placed between the 
eye and the objects intended to form the picture. 

These two plates are enclosed in a tin or paper tube, and the ob- 
jects, consisting of pieces of colored glass, beads, or other highly 

* Although bodies, from the arrangement of their particles, have a tendency 
to absorb some rays, and reflect others, they are not so uniform in their arrange- 
ment as to reflect only pure rays of one color, and perfectly absorb all others ; it 
in found, on the contrary, that a body reflects, in great abundance, the rays which 
determine its color, and the others, in a greater or less degree, in proportion as 
they are nearer or further from its color, in the order of refrangibility. Thus, the 
green leaves of a rose will reflect a few of the red rays, which will give them a 
brown tinge. Deepness, or darkness of color, proceeds from a deficiency rather than 
from an abundance of reflected rays. Thus, if a hody reflects only a few of the 
green rays, it will appear of a dark green. The brightness and intensity of a color 
shows that a great quantity of rays are reflected. That bodies sometimes change 
their color, is owing to some chemical change, which takes place in the internal 
arrangement of their parts, whereby they lose their tendency to reflect certain 
colors, and acquire the power of reflecting others. 

t The word Kaleidescope is derived from the Greek language, and means " The 
sight of a beautiful form." The instrument was invented by Dr. Brewster, of 
Edinburgh, a few years ago. 

Of what color do bodies appear in the dark? Why do some bodies appear 
differently by candle-light; What is necessary to produce color? What ex- 
periments are related to prove the truth of the above.' What rays does 
a body reflect in the greatest abundance j In what proportion does it reflect the 
other rays ? Why do the green leaves of a rose appear to have a brown tinge ? 
What does the brightness and intensity of a color show ? Why do some bodies 
change their color? 379. What is a multiplying glass ? How many times will 
an object, viewed through a multiplying glass," be multiplied ? What is the prin- 
ciple of the magnifying glass? 380. Of what does the kaleidescope consist? 
From what is the word kaleidescope derived, and what does it mean ? By whom 
was the instrument invented .' What is here said with regard to the kaleidescope ? 



152 NATURAL PHILOSOPHY. 

colored fragments, are loosely confined between two circular pieces 
of common glass, the outer one of which is slightly ground, to 
make the light uniform. On looking down the tube through a small 
aperture, and where the ends of the glass plates nearly meet, a 
beautiful circular figure will be seen, having six angles, the reflect- 
ors being inclined the sixth part of a circle. If inclined the twelfth 
part, or twentieth part of a circle, twelve or twenty angles will be 
seen. By turning the tube so as to alter the position of the colored 
fragments within, these beautiful forms will be changed ; by which 
an almost infinite variety of patterns may be produced. 



SECTION XVI. 

Electricity. 

381. The word Electricity* is a term used by phi- 
losophers to signify the operations of a very subtle and 
elastic fluid, which pervades the material world. 

382. Electricity can be seen only in its effects; which 
are exhibited in the form of attraction and repulsion. 

If a piece of amber, or sealing-wax, or a piece of smooth glass, 
perfectly clean and dry, be briskly rubbed with a dry woollen cloth, 
and immediately afterwards be held over small and light bodies, 

* This word is derived from a Greek word, which signifies amber, because this 
substance was supposed to possess, in a remarkable degree, the property of produc- 
ing the fluid, when excited or rubbed. The property itself was first discovered 
by Thales, of Miletus, one of the seven wise men of Greece. The word is dow 
used to express both the fluid itself, and the science which treats of it. 

The nature of electricity, is entirely unknown. Some philosophers consider it a 
fluid ; othfiis consider it as two fluids of opposite qualities ; and others again deny 
its materiality, and deem it, like attraction, a mere property of matter. In this 
volume the opinion of Dr. Franklin is adopted, who supposed it to be a single 
fluid, disposed to diffuse itself equally among all substances ; and exhibiting its 
peculiar effects only when a body by any means became possessed of more or less 
than its proper share. That when any substance has moie than its natural share 
it is said to be positively electrified, arid that when it has less than its natural 
Bhare it is said to be negatively electrified, — that positive electricity implies a re- 
dundancy, and negative electricity a deficiency of the fluid 



381. What is electricity ? What is stated in the note with regard to the word 
electricity ? By whom was this property fust discovered? What is stated with 
regard to the nature of electricity ? Whose and what opinion is adopted in this 
volume? When is a substance said to be po-i'ively electrified > When is it said 
to be negatively electrified ? What does positive electricity imply .» What does 
negative electricity imply i 382. How can electricity be seen? How are theae 
effects exhibited? 



ELECTRICITY. 153 

such as pieces of paper, thread, cork, straw, feathers or fragments 
of gold leaf, strewed upon a table, these bodies will be attracted, 
and fly towards the surface that has been rubbed, and adhere to it 
for a certain time. The surfaces that have acquired this power of 
attraction are said to be excited ; and the substances thus susceptible 
of being excited are called electrics, while those which cannot be 
excited in a similar manner are called nan-electrics. 

383. The science of electricity, therefore, divides 
all substances into two kinds; namely, Electrics, or 
those substances which can be excited, and Non-elec- 
trics, or those substances which cannot be excited. 

384. The electric fluid is readily communicated from 
one substance to another. Some substances, however, 
will not allow it to pass through them, while others give 
it a free passage. Those substances, through which it 
passes without obstruction, are called conductors; while 
those through which it cannot readily pass are called 
non-conductors ; and it is found, by experiment, that all 
electrics are non-conductors, and all non-electrics are 
good conductors of electricity. 

The following substances are electrics, or non-conductors of elec- 
tricity; namely, the atmospheric air, (when dry,) glass, feathers, 
amber, diamond, and all precious stones, all gums and resins, the 
oxides of all metals, bees wax, sealing-wax, sulphur, silk, wool, hair, 
paper, cotton. All these substances must be dry, or they will become 
more or less conductors. 

The following substances are non-electrics, or conductors of elec- 
tricity ; namely, all metals, charcoal, living animals, vapor or steam. 

The following are imperfect conductors, (that is, they conduct 
the electric fluid, but not so readily as the substances above mention- 
ed,) namely, water, green vegetables, damp air, wet wood, and all 
substances containing moisture, common wood, dead animals, bone, 
horn, &c. 

385. When a conductor, that is, a substance which 
can conduct electricity, is on all sides surrounded by 
non-conducting substances, it is said to be insulated. 



What illustration of this is given ? What is said of the surfaces which have 
acquired the power of attraction ! What are electrics >. What are non-electrics >. 
383. Into how many kinds does the science of electricity divide all substances! 
What are they ? 384. What is said with regard to the communication of the elec- 
tric fluid from one substance to another .' Will all substances allow it to pass 
through them .' What bodies are called conductors? What bodies are called 
non-conductors? What has been found, by experiment, with regard to electrics 
and non-electrics ' What substances are electrics or non conductors I Why must 
these substances be dry? What substances are non-electrics;tM- conductors ? What 
substances are mentioned as imperfect conductors .' 385. When is a substance 
said to be insulated .' 

14* 



154 NATURAL PHILOSOPHY.. 

As glass is a non-conducting substance, any conducting substance 
surrounded with glass, or standing on a table or stool, with glass 
legs, will be insulated. 

As the air is a non-conductor, when dry, a substance which rests 
on any non-conducting substance will be insulated, unless it com- 
municates with the ground, the floor, a table, &c. 

386. When a communication is made between a 
conductor and an excited surface, (See No. 382,) the 
electricity from the excited surface is immediately con- 
veyed by the conductor to the ground ;* but if the con- 
ductor be insulated, its whole surface will become elec- 
tric, and it is said to be charged. 

387. The principal mode of exciting electricity is by 
friction. 

Thus, if a thick cylinder of sealing-wax, or sulphur, or a glass 
tubet be rubbed with a silk handkerchief, a piece of clean flannel, 
or the fur of a quadruped, the electric fluid will be excited and may 
be communicated to other substances from the electric thus excited. 
The electricity excited in glass is called the vitreous or positive 
electricity ; and that obtained from sealing wax, or other resinous 
substances, is called resinous or negative electricity. 

388. The vitreous and resinous, or, in other words, 
the positive and negative electricities always accompa- 

* The earth may be considered as the principal reservoir of electricity ; and 
when a communication exists, by means of any conducting substance, between & 
body containing more than its natural share of the fluid, and the earth, the body 
will immediately lose its redundant quantity, and the fluid will escape to the earth. 
Thus, when a person holds a metallic tube to an excited surface, the electricity 
escapes from the surface to the tube, and passes from the tube through the person 
(as living animals are good conductors) to the floor ; and the floor being connected 
with the earth by conducting substances, siidi as the timbers. &c which support 
the building, the electricity will finally pass off by a regular succession of conduct- 
ing substances from the excited surface to the earth. But if the chain of conduct- 
ing substances be interrupted — that is, if any non-conducting substance occurs be- 
tween the excited surface and the course which the fluid takes in its progress to 
the earth, the conducting substances will be insulated, and become charged with 
electricity. Thus, if an excited surface be connected by a long chain to a metal- 
lic tube, and the metallic tube be held by a person who is standing on a stool with 
glass legs, or on a cake of sealing-wax, resin, or any other nonconducting sub- 
stance, the electricity cannot pass to the ground, and the person, the chain and 
the tube will all become electrified. 

f Whatever substance is used, it must be perfectly dry. If, therefore, a glass 
tube be used, it should previously be held to the fire, and gently warmed, in order 
to remove all moisture from its surface. 

386. When a communication is made between a conductor and an excited sur- 
face, wlvre is the electricity from the excited substance conveyed? When is it 
said to b;; charged ? When a communication exists by means of any conducting 
substance, between a body containing more than its natural share of the fluid 
and the earth, what will become of the ledundant quantity which the body pos- 
sesses? What illustration of this is given ? What follows if this chain of con- 
ducting substances be interrupted ; 3S7. What is the principal mode of exciting 
electricity! What illustration of this is given? What is the electricity excited 
in glass called? What is that obtained from resinous substances called? 388. 
What is stated in No. 388 with regard to positive and negative electricity ? 



ELECTRICITY 



155 



ny each other; for if any surface become positive, the 
surface vviih which it is rubbed will become negative; 
and if any surface be made positive, the nearest con- 
ducting- surface will become negative. And if positive 
electricity be communicated to one side of an electric, 
(as a pane of glass, or a glass phial,) the opposite side 
will become pegatively electrified, and the plate or the 
glass is then said to be charged. 

When one side of a metallic, or other .conductor, receives the 
electric fluid, its whole surface is instantly pervaded; but when an 
electric is presented to an electrified body, it becomes electrified in a 
small spot only. 

"When two surfaces oppositely electrified are united, their powers 
are destroyed; and if their union be made through the human body, 
it produces an affection of the nerves, called an electric shock. 

Bodies that are charged with the same kind of electricity, appear 
to repel each other; but if one have more and the other less than 
its share, they will first attract one another, until the equilibrium is 
restored, and then repel each other. 

389. The Leyden jar is a glass vessel used for the 
purpose of accumulating the electric fluid, procured 
from excited surfaces. 

Fig. 117 represents a Leyden jar. It is a glass ves- 
sel or phial coated both on the inside and the out- 
side with tin foil. It is provided with a cork or 
wooden stopper through which a metallic rod pass- 
es, terminating in a brass knob or ball at the top, 
and connected by means of a wire, at the other 
end, with the inside coating of the jar. The coat- 
ing extends both on the inside and outside only to 
within two or three inches of the top, or the stop- 
per. Thus prepared, when an excited surface is 
applied to the brass knob, or connected with it by 
means of a chain or any conducting surface, it 
parts with its electricity, and the fluid enters the 
jar, which is then said to be charged. 

When the Leyden jar is charged, the fluid is con- 
tained in the inside coating of the phial ; and as 
this coating is insulated, the fluid will remain in 
the jar until a communication is made by means of 
some conducting substance, between the inside and 
the outside of t e jar. If then a person apply one hand or finger 
to the brass knob, and the other to the outside coatmg of the jar, a 



Fig. 117. 




What follows when one side of a metallic, or other conductor, receives the elec- 
tric fluid ! What follows when an electric is presented to an electrified body? 
What follows when two surfaces, oppositely electrified, ate united ? When do 
electric bodies repel each other ! When do they attract each other I 389. For 
what is the Leyden jar used \ What does fig. 117 represent ? W T hat is a Leyden 
iar ? When is the jar said to be charged ! How can the jar be discharged ? 



156 NATURAL PHILOSOPHY. 

communication will be formed by means of the brass knob with the 
inside and outside of the jar, and the jar will be discharged. A 
phial or jar that is insulated cannot be charged. 

390. An electrical battery is composed of a number 
of Leyden jars connected together. The inner coat- 
ings of the jars are connected together by chains or me- 
tallic bars attached to the brass knobs of each jar ; and 
the outer coatings have a similar connexion established 
by placing the phials on a sheet of tin foil. The whole 
battery may then be charged like a single phial, or jar. 
For the sake of convenience in discharging the battery, 
a knob, connected with the tin foil on which the jars 
stand, projects from the bottom of the box which con- 
tains the jars. 

391. The jointed discharger is an instrument used to 
discharge ajar, or battery. 

Illustration. Fig. 118 represents the jointed discharger. It con- 
sists of two rods, generally of brass, terminating at one end in brass 
Fi s . us. balls, and connected together at 

the other end by a joint, like that 
of a pair of tongs, allowing 
them to be opened or closed. It 
is furnished with a glass handle, 
to secure the person who holds 
it from the effects of a shock. 
When opened, one of the balls 
is made to touch the outside 
coating of the jar. or the knob 
connected with the bottom of 
the batteiy, and the other is quickly applied to the knob of the jar, 
or jars. A communication being thus formed, between the inside 
and the outside of the jar, a discharge of the fluid will be produced. 
When a charge of electricity is to be sent through any particular 
substance, the substance must form a part of the circuit of the elec- 
tricity, as it is termed ; that is, it must be placed in such a man- 
ner that the fluid cannot pass from the inside to the outside surface 
of the jar, or battery, without passing through the substance in its 
passage. 

If the balls be removed from the jointed discharger, and the two 
rods terminate in sharp points, the electricity will pass off silently 
and produce but little effect. 

Can an insulated jar be charged ? 390. Of what is an electrical battery com- 
posed I How are the inner coatings of the jars connected together i How are the 
outer coatings connected ? In what way is the battery charged. 391. What is 
the jointed discharger .' What does fig. 1 18 represent ? Of what does it consist ? 
What is necessary when a charge of electricity is to be sent through any partic- 
ular substance? How can the electricity be made to pass off silently ? 




ELECTRICITY. 157 

392. Metallic rods, with sharp points, silently attract 
the electric fluid. 

A Leyden jar, or a battery, may be silently discharged by holding 
the finest needle in the hand towards the knob. It is on this princi- 
ple that lightning-rods are constructed. The electric fluid is silent- 
ly drawn from the cloud by the sharp points on the rods, and is 
thus prevented from suddenly exploding on high buildings. 

Electricity, of one kind or the other, is generally induced in sur- 
rounding bodies by the vicinity of a highly-excited electric. This 
mode of communicating electricity by approach, is styled induction. 

Any body, capable of free motion, on approaching another body, 
powerfully electrified, will be thrown into a contrary state of elec- 
tricity. Thus, a feather, brought near to a. glass tube excited by fric- 
tion, is attracted by it ; and, therefore, previously to its touching the 
tube, negative electricity must have been induced in it. On the con- 
trary, if a feather be brought near to excited sealing-wax. it will be 
attracted, and, consequently, positive electricity must have been in- 
duced in it before contact. 

When electricity is communicated from one body to another in 
contact with it, it is called electricity by transfer. 

393. The electrical machine is a machine construct- 
ed for the purpose of accumulating or collecting elec- 
tricity, and transferring it to other substances. 

Electrical machines are made in various forms, but all on the 
same principle, namely, the attraction of metallic points. The elec- 
tricity is excited by the friction of silk on a glass surface, assisted by 
a mixture or preparation called an amalgam.* The glass surface is 
made either in the form of a cylinder or a circular plate, and the ma- 
chine itself is called a cylinder or a plate machine, according as it 
is made, with a cylinder or a plate-f 

* The amalgam is composed of mercury, tin and zinc. That recommended by 
Singer, is made by melting together one ounce of tin and two ounces of zinc, 
which arc to be mixed, while fluid, with six ounces of mercury, and agitated in an 
iron, or thick wooden box. until cold. It is then to be reduced to a very fine pow- 
der in a mortar, and mixed with a sufficient quantity of lard to form it into a 
paste. 

f The electrical machine described in fig. 119 is a plate machine, and an exact 
representation of the one belonging to the " Boston School Set." For one of its 
size, it is a machine of very great power, and, together with the other implements 
belonging to the same set, was constructed by Messrs. A. &. D. Davis, of this 
city. It is entirely insula ted, so that either positive or negative electricity may 
be obtained from it. 



392. In what way do metallic rods, with sharp points, attract the electric fluid? 
Upon what principle are lightning-rods constructed; When is electricity said to 
be communicated by induction ? VVh^n, by transfer ? 393. For what purpose is 
the electneal machine constructed ? Upon what principle are all electrical ma- 
chines constructed .' How is the plectricity excited ? Of what is the amalgam 
composed ? In what form is the glass surface made ? When is the machine calt. 
od a plate machine > When is it called a cylinder machine ? 



158 



NATURAL PHILOSOPHY. 



Fig. 119 represents a plate electrical machine. A D is the stand 
of the machine, L L L L are the four glass legs, or posts which sup- 
port and insulate the parts of the machine. P is the glass plate, 
(which in some machines is a hollow cylinder,) from which the 
electricity is excited, and H is the handle by which the plate (or cyl- 
inder) is turned. R is a leather cushion, or rubber, held closely to 
both sides of the glass plate by a brass clasp, supported by the post 
G L, which is ealled the rubber post. S is a silk ba?,* embraced 
by the same clasp that holds the leather cushion or rubber ; and it 
is connected by strings S S S attached to its three other corners. 

Fig. 119. 



bO « 




to the legs L L and the fork F of the prime conductor. C is the 
prime conductor, terminating at one end with a moveable brass ball, 
B, and at the other by the forkF, which has one prong on each side 
of the glass plate. On each prong of the fork there are several 
sharp points projecting towards the plate, to collect the electricity 
(See No. 392,) as it is generated by the friction of the plate against 
the rubber. V is a chain or wire attached to the brass ball on the 
rubber post, and resting on the table or the floor, designed to con- 
vey the fluid from the ground to the plate. When negative electrici- 
ty is to be obtained, this chain is removed from the rubber post, and 
attached to the prime conductor, and the electricity is to be gathered 
from the ball on the rubber post. 

The operation of tlie machine is as follows: By turning the handle 
H the glass plate is pressed by the rubber. The friction of the rub- 
ber against the glass plate (or cylinder) produces a transfer of the 
electric fluid from the rubber to the plate ; that is, the cushion be- 
comes negatively and the glass positively electrified. The fluid 
which thus adheres to the glass, is carried round bv the revolution 
of the cylinder; and its escape being prevented by the silk bag, or 
flap, which covers the plate (or cylinder) until it comes to the im- 

*In cylindrical machines this silk bag is called " thefiap." 



What does figure 119 represent » Explain the figure. Explain 
of the machine. 



operation. 



ELECTRICITY. 159 

mediate vicinity of the metallic points, on the fork F, it is attracted 
by the points, and carried by them to the prime conductor. Posi- 
tive electricity is thus accumulated in the prime conductor, while 
the conductor on the rubber post, being deprived of this electricity, 
is negatively electrified. The fluid may then be collected by a Ley- 
den jar, from the prime conductor, or conveyed, by means of a chain 
attached to the prime conductor, to any substance which is to be 
electrified. If both of the conductors are insulated, but a small por- 
tion of the electric fluid can be excited; for this reason, the chain 
must in all causes be attached to the rubber post, when positive electricity 
is required, and to the prime conductor, when negative electricity is 
wanted. 

Experiments with the Electrical Machine. 

1. On the prime conductor of the electrical machine is placed the 
electrometer,* E. It consists of a wooden ball mounted on a metal' 
lie stick, or wire, having two pith balls, suspended by silk or hair. 
When the machine is worked, the pith balls, being repelled, fly 
apart, as is represented in the figure; and they will continue elevat- 
ed until the electricity is drawn off. But if an uninsulated conduct- 
ing substance touch the prime conductor, the pith balls will fall. The 
height to which the balls rise, and the quickness with which they 
are elevated, afford some test of the quality of the machine. 

2. The balls of the electrometer, when elevated, are attracted by 
an excited piece of sealing-wax or resin, and repelled by a piece 
of excited glass. [See No. 382.J 

3. If an electric, or a non-conductor, be presented to the prime 
conductor, when charged, it will produce no effect on the balls ; but 
if a non-electric, or any conducting substance be presented to the 
conductor, the balls of the electrometer will fall. This shows that 
the conductor has parted with its electricity, and that the fluid has 
passed off to the earth through the substance, and the hand of the 
person presenting it. 

4. When the machine is turned, if a person touches the prime 
conductor, the fluid passes off through the person to the floor with- 
out his feeling it. But if he present his finger, his knuckle, or any 
part of the body, near to the conductor, without touching it, a spark 
will pass from "the conductor to the knuckle, which will produce a 
sensation similar to the pricking of a pin or needle. 

5. If a person stand on a stool with glass legs, or any other non- 
conductor, he will be insulated. If in this situation he touches the 

* The word : ' electrometer " means "a measurer of electricity. " It is made 
in a variety of forms It sometimes consists of a single pith ball, attached to a 
light rod, in thp manner of a pendulum, before a graduated arc or circle. An elec- 
troscope is an instrument of more delicate construction, to detect the presence of 
electricity. 



To what must the chain be attached when positive electricity is required ? To 
what must it be attached when negative electricity is wanted ? 1. What is the 
first experiment, mentioned, with the electrical machine? What does the word 
electrometer mean ! Of what does it sometimes consist? What is an electro- 
scope? 2. What is the second experiment > 3. What is the third ? What doei 
this show ? 4. What is the fourth I 5. What is the fifth ? 



160 NATURAL PHILOSOPHY. 

prime conductor, or a chain connected with it, when the machine is 
worked, sparks may be drawn from any part of the body in the same 
manner as from the prime conductor. While the person remains 
insulated, he experiences no sensation from being filled with elec- 
tricity ; or, if a metallic point be presented to any part of his body, 
the fluid may be drawn off silently, without being perceived. But 
if he touch a blunt piece of metal, or any other conducting sub- 
stance, or if he steps from the stool to the floor, he will feel the elec- 
tric shock ; and the shock will vary in force according to the quan- 
tity of fluid with which he is charged. 

6. The Leyden jar may be charged by presenting it to the prime 
conductor, when the machine is worked. If the ball of the jar louch 
the prime conductor, it will receive the fluid silently ; but if the ball 
of the jar be held at a small distance from the prime conductor, the 
sparks will be seen darting from the prime conductor to the jar with 
considerable noise. 

The jar may in like manner be filled with negative electricity, by 
applying it to the ball on the rubber part, and connecting the chain 
with the prime conductor. 

7. If the Leyden jar is charged from the prime conductor, (that is, 
with positive electricity) and presented to the pith balls of the elec- 
trometer, they will be repelled ; but if the jar is charged from the 
brass ball of the rubber post, (that is, with negative electricity) they 
will be attracted. 

8. If the electrometer be removed from the prime conductor, and a 
pointed wire be substituted for it, a wire with sharp points bent in 
the form of an S, resting upon it, will be made to revolve rapidly. 
In a similar manner the motion of the sun and the earth around 
their common centre of gravity, together with the motion of the 
earth and the moon may be represented.* 

9. If powdered resin be scattered over dry cotton wool, loosely 
wrapped on one end of the jointed discharger, it may be inflamed by 
the discharge of the battery or a Leyden jar. Gunpowder may be 
substituted for the resin. 

Fig. 120. 10. The universal discharger, 

represented in figure 120, is 
an instrument for directing a 
charge of electricity through 
any substance, with certainty 
and precision. It consists of 
two sliding rods, A B and C D, 
terminating at the extremities, 
A and D, with brass balls, and 
at the other ends, which rest 
upon the ivory table or stand E } 

*In the electrical department of the "Boston School Set," there is a brass 
wire in the form of an S, as above described, together with brass balls, mounted 
on wires, to represent the sun, earth, and moon, revolving around their common 
centre of gravity. 

6. What is the sixth >. How may the jar be filled with negative electricity ? 
7. What is the seventh > 8. What is the eighth; 9. What is the ninth? 10. 
What figure represents the universal discharger? What is its use J Uf what 
does it consist ? 




ELECTRICITY. 



161 



i 



having a fork, to which any small substance may be attached. The 
whole is insulated by glass legs or pillars. The rods slide through 
collars, by which means their distance from one another may be ad- 
justed. 

In using the universal discharger, one of the rods or slides must 
be connected by a chain, or, otherwise, with the outside, and the 
other with the inside coating of the jar or battery. By this means 
the substance through which the charge is to be sent is placed with- 
in the electric circuit. (See No. 391, Illustration.) 

By means of the universal discharger, a piece of a watch-spring, 
or any other small metallic substance" may be burnt. The substance 
must be placed in the forks of the slides, and the slides placed with- 
in the electric circuit, in the manner described in the last paragraph. 
In the same manner, by bringing the forks of the slides into contact 
with a substance placed upon the ivory stand of the discharger, such 
as an egs;, a piece of a potato, water, &c. it may be illuminated. 

11. The electrical bells, represented in figure 121, are designed to 
show the effects of electrical attraction and repulsion. They are 
Fic». i2i. thus to be applied. The ball 

B of the prime conductor, 
with its rod, is to be unscrew- 
^^f ed, and the rod on which the 

bells are suspended is to be 
screwed in its place. The 
middle bell is to be connected 
by a chain, with the table or 
i the floor. When the ma- 
» chine is then slowly turned, 
the balls suspended between 
the bells will be alternately 
attracted and repelled by the 
• bells, and cause a constant 

ringing. If the battery be 
charged and connected with the prime conductor, the bells will con- 
tinue to ring until all the fluid from the battery has escaped. 

It may be observed that the fluid from the prime conductor passes 
readily from the two outer bells, which are suspended by chains ; they, 
therefore, attract the two balls towards them. The balls becoming 
electrified by contact with the outer bells, are repelled by them and 
driven to the middle bell, to which they communicate their electrici- 
ty ; having parted with their electricity they are repelled by the mid- 
dle bell, and again attracted by the outer ones, and thus the constant 
ringing is maintained. The fluid which is communicated to the 
middle bell, is conducted to the earth by the chain attached to it. 

12. Ether, or spirits of wine, may be inflamed by a spark com- 
municated from a person, in the following manner. The person 
standing on the insulating stool, (that is, the stool with glass legs,) 



What is necessary in using the universal discharger.' What is effected by this 
means? What experiments are shown by means of the universal discharger? 
How must the substance be placed ! 11. What fig. represents the electrical bells? 
What are they designed to show? How are they to be applied I What farther 
may be observed with regard to this last experiment .' 12. What is the twelfth 
experiment mentioned .' 

15 



162 



NATURAL PHILOSOPHY. 



receives the electric fluid from the prime conductor, by touching 
the conductor or any conducting substance in contact with it, he then 
inserts the knuckles of his hand in a small quantity of sulphuric 
ether, or spirits of wine, held in a shallow metallic cup, by another 
person, who is not insulated, and the ether or spirits immediately 
inflames. In this case the fluid passes from the conductor to the per- 
son who is insulated, and he becomes charged with electricity. As 
soon as he touches the liquid in the cup, the electric fluid, passing 
from him to the spirit, sets it on fire. 

13. The passage of the electric fluid from one conducting sub- 
stance to another, is beautifully exhibited by means of a glass tube 
having a brass ball at each end, and coated in the inside with small 
pieces of tin foil, placed at small distances from each other in a spi- 

Fiff. 122. 



o— QUE 



szhu) — o 



a 



ral direction, as represented in figure 192. This is called the spiral 

tube. 

In the same manner various figures, letters, and words may be 

Fig. 123. represented, by arranging similar pieces of 

tin foil between two pieces of flat glass. 

These experiments appear more brilliant in 

a darkened room. 

14. Fig. 123 represents the hydrogen can- 
non or pistol. When filled with hydrogen 
gas,* if the insulated knob K be presented 
to the prime conductor, it will immediately 
explode. 



* Connected with " the Boston School Set" of philosophical 
apparatus, is an an icle called by the manufacturer "a gasom- 
eter," but which is more properly a gas generator. It is repre- 
sented in Fig. 124. It consists of a glass vessel, with a brass 
cover, on the centre of which is a stop cock •, from the inside 
of the cover, another glass vessel is suspended with its 
open end downwards. Within this, a large piece of zinc is 
suspended by a wire. The outer vessel contains a mixture 
of sulphuric acid and water, about nine parts of water to 
one of acid. When the cover, to which the inner glass is 
firmly fixed, is placed upon the vessel, the acid acting upon 
the zinc, causes the metal to absorb the oxygen of the wa- 
ter, and the hydrogen, the other constituent part of the wa- 
ter (See Illustration 1, under JVo. 38., page 12.) being thus 
disengaged, rises in the inner glass, from which it expels 
the water ; and when the stop cock is turned the hydrogen 
gas may be collected in the hydrogen pistol, or any other 
vessel. 




13. What is the thirteenth ! What does fig. 1 
fig. 123 represent ! When will the pistol explode : 
Of what does it consist .' 



2 represent? 14. What does 
What does fig. 124 represent? 



ELECTRICITY. 



163 




15. Fig. 125 represents 
the electrical sports- 
man. From the larg- 
er ball of a Leyden 
jar two birds made of 
pith* are suspended by- 
silk or hair. When the 
jar is charged the birds 
will rise, as represented 
in the figure, on ac- 
count of the repulsion 
of the fluid in the jar. 
If the jar be then 
placed on the tin foil of 
the stand, and the smaller ball placed within a half inch of the end 
of the gun, a discharge will be produced, and the birds will fall. 

16 If images, made of pith, or small pieces of paper, are placed 
under the insulated stool, (that is, the stool with glass legs,) and a 
connexion be made between the prime conductor and the top of the 
stool, the images, &c. will be alternately attracted and repelled ; or, 
in other words> they will first rise to the electrified top of the stool, 
and thus becoming themselves electrified, by contact with the elec- 
trified top of the stool, they will then be repelled, and fall to the 
ground, the floor, or the table ; where, parting with their electricity, 
they will again be attracted by the stool, thus rising and falling with 
considerable rapidity. In order to conduct this experiment success- 
fully the images, &c. must be placed within a short distance of the 
bottom of the stool. 

17. The straighi receiver, connected with the pneumatic set, rep- 
resented in figure 78, and described on pasre 86, as is there mentioned, 
is a jar coated with parallel strips of tin foil. Let this be charged, by 
placing the inside in contact with the prime conductor, and taming 
it round so that each strip may successively touch the ball of the 
conductor. If a number of pith balls be then placed within the 
glass, or the glass be placed over the pith balls, they will bound 
rapidly up and down, and their motions will be repeated, as often as 
the glass is touched by the hand, until the jar or glass has parted 
with its electricity. This experiment may also be performed by a 
plain glass tumbler. 

18. A hole may be perforated through a quire of paper, by charg- 
ing the battery, resting the paper upon the brass ball of the battery, 
and making a communication, by means of the jointed discharger, 
between the ball of one of the jars and the brass ball of the box. 
The paper, in this case, will be between the ball of the battery and 
the end of the discharger. 

*This substance is produced in large quantities from the corn stalk, the whole 
of which, with the exception of the outside, is composed of pith. 



15. What does fig. 125 represent i When will the birds rise I Why will they 
rise > When will the birds fall ! 16. What is the sixteenth experiment which is 
here mentioned ? How must the images be placed to conduct the experiment suc- 
cessfully ? 17. What is the seventeenth experiment I 18. What is the eighteenth 
experiment » 



164 



NATURAL PHILOSOPHY. 




19. The thunder house, Fig. 126, is designed to show the security 
afforded by lightning rods, when lightning strikes a building. (See 
Fig. 126. No. 392.) This is done by placing a 

highly eombustib'e material in the in- 
side of the house, and passing a charge 
of electricity through it. On the floor of 
the house is a surface of tin foil. The 
hydrogen pistol, (See Fig. 123.) being 
filled with hydrogen gas from the gas- 
ometer, (See Fig. 124,) must be placed 
on the floor of the thunder house, and 
connected with the wire on the oppo- 
site side. The house being then put 
together, a chain must be connected 
with the wire on the side opposite to the lightning rod, and placed in 
contact either with a single Leyden jar or with the battery- When 
the jar, thus situated, is charged, if a connexion be formed between 
the jar and the points of the lightning rod, the fluid will pass off 
silently, and produce no effect. But if a small brass ball be 
placed on the points of the rod, and a charge of electricity be sent 
to it, from the jar or the battery, the gas in the pistol will ex- 
plode, and throw the parts of the house asunder with a loud noise * 
20. If the ball of the prime conductor be removed and a pointed 
wire be put in its place, the current of electricity flowing from the 
point, when the machine is turned, may be perceived by placing a 
lighted lamp before it; the flame will be blown from the point; and 
this will be the case in what part soever of the machine the point is 
placed, whether on the prime conductor or the rubber; or if the 
point be held in the hand and the flame placed between it and the 
machine, thus showing that in all cases the fluid is bio tvn from the 
point. Delicate apparatus may be put in motion by the electric flu- 
id when issuing from a point. In this way electrical orreries, mills, 
&c. are constructed. [See No. 9, under Electrical Experiments.] 



*The success of this experiment depends upon the proper connexion of the jar 
with the lightning rod, and the electrical pistol. On the side of the house opposite 
to the lightning rod there is a wire, passing through the side, and terminating on 
the outside in a hook. When the house is put together, this wire, in ihe inside, 
must touch the tin foil on the floor of the house. The hydrogen pistol must stand 
on the tin foil, and it3 insulated knob or wire, projecting from its side, must he 
connected with the lower end of the lightning rod extending into the inside of the 
house. A communication must then be made between the hook on the outside of 
the house, and the outside of the jar or battery. This is conveniently done by at- 
taching one end of a chain to the hook and holding the other end in the hand 
against the side of a charged jar. By present ing the knob of the jar to the points 
of the lightning rod noeffect is produced, but if a brass ball he placed on the points 
atP, and the knob of the jar be presented to the ball, the explosion will take place. 

The thunder house belonging to " the Boston School Set " is held together by 
magnets attached to the inner surface of the sides. 



19. What does fig. 126 represent ? What is it designed to show ? How is this 
done? When will the fluids pass off silently and produce no effect.' When will 
there be an explosion and the house be torn asunder ' Upon what does the success 
of this experiment depend? What is said in the note with regard to the thunder 
house.' 20. What is the twentieth experiment which is here mentioned >. In what 
way are electrical orreries, mills, &c. constructed ? 



ELECTRICITY. 165 

21. If the electrometer be removed from the prime conductor, and 
a tuft of feathers or hair, fastened to a stick or wire, be put in its 
place, on turning the machine the feathers or hair will become elec- 
trified, and the separate hairs will rise and repel each other. A toy 
is in this way constructed, representing a person under excessive 
fright. On touching the head with the hand, or any conducting 
substance, not insulated, the hair will fall. 

22. Gold leaf may be forced into the pores of glass by placing it 
between two slips of window glass, pressing the slips of glass firmly 
together, and sending a shock from a battery through them. 

23. If gold leaf be placed between two cards, and a strong charge 
be passed through them, it will be completely fused. 

24. When electricity enters at a point, it appears in the form of a 
star ; but when it goes out from a point, it puts on the appearance 
of a brush. 

394. Lightning is the rapid motion of vast quantities 
of electric matter — and thunder is the noise produced 
by the rapid motion of lightning through the air. 

395. The aurora borealis, (or northern lights,) is sup- 
posed to be caused by the electric fluid passing through 
highly rarified air; and most of the great convulsions 
of nature, such as earthquakes, whirlwinds, hurricanes, 
water-spouts, &c. are generally accompanied by elec- 
tricity, and often depend upon it.* 

*The experiments which have now been described exemplify all the elementary 
principles of the science of electricity. These experiments may be varied, multi- 
plied and extended in innumerable forms, by an ingenious practical electrician. 
Among other things with which the subject may he made interesting, may be men- 
tioned the following facts, &c. 

A number of feathers, suspended by strings from an insulated conducting sub- 
stance, will rise and present the appearance cf a flight of birds. As soon as the 
substance is discharged the feathers will fall. The experiment described 
in No. 15. rig. 125, page 163, may be varied by placing the sportsman on the 
prime conductor, without the use of the Leyden jar, to which the birds are at- 
tached; 

The experiment in No. 16 may be varied by the use of two plates of metal, one 
of which may be suspended from the prime conductor and the painted images 
placed between them. 

Instead of the Loyden jar a plate of common glass, (a pane of window glass, for 
instance,) may be coated on both sides with tin foil, leaving the edges bare. A 
bent wire balanced on the edge of the glass, to the ends of which balls may be at- 
tached, with an image at each end, may be made to represent two persons, tilting, 
on the same principle by which the electrical bells are made to ring. [See No. 
11, page 161, fig. 121.] 

A beautiful little saw mill was lately exhibited at a lecture at the Odeon, in 
this city, by Mr. Quimby, its ingenious contriver. The moving power was a 
wheel, with balls at the ends of the spokes, situated within the attractive influ- 
ence of two larger balls, differently electrified. As the balls on the spokes were 
attracted by one of the larger balls, they changed their electrical state and were 
ttttracted by the other, which, in its return, repelled them, and thus the motion 

21. What is the twenty-first experiment i 22. What is the twenty-second I 
23. What is the twenty-third? 24. What is the twenty-fourth? 394. What is 
lightning? What is thunder? 395. How is the aurora borealis supposed to be 
caused >. 

15* 



166 NATURAL PHILOSOPHY. 

The electricity which a body manifests by being brought near to 
an excited body, without receiving a spark from it, is said to be ac- 
quired by induction. When an insulated but unelecirifled conductor 
is brought near an insulated charged conductor, the end near to the 
excited conductor assumes a state of opposite electricity, while the 

being given to the wheel was communicated by cranks at the end of the axle to 
the saws above. 

When the hand is presented to the prime conductor, a spark is communicated, 
attended with a slightly painful sensation. But if a pin or a needle lie held in the 
hand with the point towards the conductor, neiiher spark nor pain will be perceiv- 
ed, owing to the attracting, ( or perhaps, more properly speaking, the receiving) 
power, of the point. 

That square rods are better than round ones to conduct electricity silently to 
the ground and thus to protect buildings, may be proved by causing each kind of 
rod to approach the prime conductor when charged. It will thus be perceived that 
while little effect is produced on the pith balls of theelectiometer by the near ap- 
proach of the round rod, on the approach of the square one the balls will im- 
mediately fall. The round rod also, will produce an explosion and a spark, from 
the ball of the prime conductor, while the square one will draw off the fluid si- 
lently. 

The effects of pointed conductors upon clouds charged with electricity may be 
familiarly exemplied by suspending a small fleece of cotton wool from the prime 
conductor, and other smaller fleeces from the upper one, by small filaments. On 
presenting a point to them they will be repelled and all drawn together ; but if a 
blunt conductor approach them they will be attracted. 

From a great variety of facts, it has been ascertained that lightning rods afford 
but little security to any part of a building beyond twenty feet from them ; and 
that when a rod is painted it loses its conducting power. The lightning rods of 
the most approved construction, and in strictest accordance with philosophical 
principles, are composed of small square rods, (similar to nail rods.) They run 
over the building, and down each of the corners, presenting many elevated points 
in their course. At each of the corners, and on the chimneys, the rods are ele- 
vated several feet above the building. Rods of this description have been erected 
on all the public school houses and other public buildings of this city, by order of 
the city authorities. Thpy were constructed by Dr. King. Air. Cuimby, of 
Charlestown, has introduced an improvement on the rods of Dr. King by twisting 
the square rods, and thus multiplying the sharp surfaces presented to collect the 
fluids. 

The removal of silk and woollen garments, worn during the day in cold weather 
is often accompanied by a slight noise resembling that of sparks issuing from a 
fire. A similar effect is produced on passing the~hand softly over the back of a 
cat. These effects are produced by electricity. 

It may here be remarked that the terms positive and negative are merely rela- 
tive terms as applied to the subject of electricity. Thus, a body which is possess- 
ed of its natural share of electricity is positive in respect to one that has less, and 
negative in respect to one that has more than its natural share of the fluid. So, 
also, one that has more than its natural share is positive with regard to one that 
has only its natural share, or less than its natural share — and negative in respect 
to one having a larger share than itself. 

The experiments with the spiral tube, page 162, may be beautifully varied by 
having a collection of such tubes placed on a stand ; and a jar coated with small 
strips resembling a brick wall, presents, when it is charged, a beautiful appear- 
ance in the dark. 

The electric fluid occupies no perceptible space of time in its passage through its 
circuit. It always seems to prefer the shortest passage, when the conductois are 
equally good. Thus, if two, ten, a hundred, or a thousand or more persons, join 

Why are square rods better than round ones to conduct electricity silently to 
theground, and thus protect buildings from lightning? How far beyond the rod 
do lightning rods afford protection? In what way arc the most approved light- 
ning rods constructed? What is remarked with regard to the terms negative and 
positive? How can this be illustrated ? What is said with regard to the time 
the electric fluid occupies in its passage through its circuit? By what is the elec- 
tricity which a body manifests by being brought near to an excited body without 
receiving a spark from it, said to be acquired ? When an iusulat d, but unelectri- 
fied conductor, is brought near an insulated charged conductor, what is said with 
regard to the end near the excited conductor i 



ELECTRICITY. 167 

farther end assumes the same kind of electricity — that is, if the 
conductor is electrified positively, the unelectrified conductor will be 
negative at the nearer end and positive at the further end, while the 
middle point evinces neither positive nor negative electricity. 

hands and be made part of the circuit of the fluid in passing from the inside to the 
outside of a Loyden jar, they will all feel the shock at the same moment of time. 
But, in its passage, the fluid always prefers the best conductors. Thus, if two 
clouds, differently electrified, approach one another, the fluid, in its passage from 
one cloud to the other, will sometimes take the earth in its course, because the air 
is a bad conductor. 

In thunder storms, the electric fluid sometimes passes from the clouds to the 
earth, and sometimes from the earth to the clouds; and sometimes, as has just 
been stated, from one cloud to the earth, and from the earth to another cloud. 

It is not safe, during a thunderstorm, to take shelter under a tree, because the 
tre« attracts the fluid, and the human body being a better conductor than the tree, 
the fluid will leave the tree and pass into the body. 

It is equally dangerous to hold in the hand edge tools, or any sharp point which 
will attract the fluid. Carpenters have lost their lives during a thunder storm by 
carrying edge tools. An instance of this kind occurred in this city about ten years 
ago. A carpenter imprudently went to the window of a house, where he was at 
work, during a thunder storm, with a chisel in his hand. The sharp edge of the 
tool attracted the fluid and he was instantly killed. 

The safest position that can be chosen during a thunder storm is a recumbent 
posture on a father bed ; and in all situations a recumbent is safer than an erect 
position. No danger is to be apprehended from lightning when the interval be- 
tween the flash and the noise of the explosion is as much as three or four seconds. 
This space of time may be conveniently measured by the beatings of the pulse, if 
no time piece is at hand. 

Lightning rods were first proposed by Dr. Franklin, to whom is also ascribed 
the honor of the discovery that thunder and lightning are the effects of electricity. 
He raised a kite, constructed of a silk handkerchief adjusted to two light strips of 
cedar, with a pointed wire fixed to it ; and fastening the end of the twine to a key, 
and the key, by means of a piece of silk lace, to a post, (the silk lace serving to 
insulate the whole apparatus,) on the approach of a thunder cloud, he was able to 
collect eparks from the key, to charge Leyden jars, and to set fire to spirits. This 
experiment established the identity of lightning and electricity. The experiment 
was u dangerous one, as was proved in the case of Professor Richman, of St. Pe- 
tersburgh, who fell a sacrifice to his zeal for electrical science, by a stroke of light- 
ning from his apparatus. 

Among the most remarkablo facts, connected with the science of Electricity, 
maybe mentioned the power possessed by certain species of fishes of giving shocks, 
similar to those produced by the Leyden jar. There are three animals possessed of 
this power, namely, the Torpedo, the Gymnotus Electricus, (or Surinam Eel,) and 
the Silurus Electricus. But although it has been ascertained that the Torpedo is 
capable of giving shocks to the animal system, similar to those of the Leyden jar, 
yet he has never been made to afford a spark, nor to produce the least effect upon 
the most delicate electrometer. The Gymnotus gives a small but perceptible 
spark. The electrical powers of the Silurus are inferor to those of the torpedo" 
or the gymnotus, but still sufficient to give a distinct shock to the human system. 
This power seems to have been bestowed upon these animals to enable them to 
secure their prey ; and to resist the attacks of their enemies. Small fishes, when 
put into the water where the gymnotus is kept, are generally killed or stunned 
by the shock and swallowed by the animal, when he is hungry. The gymnotus 
seems to be possessed of a new kind of sense, by which he perceives whether the 
bodies presented to him are conductors or not. 

What example is given to illustrate this i What example is given to show that 
the fluid prefers the best conductors > In what different ways does the electric 
fluid sometimes pass in thunder storms? Why is it unsafe, during a thunder 
storm, to take shelter under a tree, or to hold in the, hand any edge tools? What 
position is the safest in a thunder storm? When is there no danger to be appre- 
hended from the lightning I By whom were lightning rods first proposed? Who 
first discovered that thunder and lightning are the effects of electricity > In what 
way did he prove this .' What is related as among the most remarkable facts con- 
nected with the science of electricity ? 



168 NATURAL PHILOSOPHY. 

SECTION XVII. 

Galvanism, or Voltaic Electricity. 

396. Galvanism is a branch of Electricity, which de- 
rives its name from Galvani,* who first discovered it. 
Electricity is produced by the mechanical action of 
bodies on one another ; but Galvanism, or Galvanic 
Electricity is produced by their chemical action. 

397. The motion of the electric fluid excited by gal- 
vanic power, differs from that explained in the sci- 
ence of electricity, in its duration; for while the latter 
exhibits itself in sudden and intermitted shocks and 
explosions, the former continues in constant and unin- 
terrupted action. 

398. The nerves and muscles of animals are most 
easily affected by the galvanic fluid ; but the voltaic or 
galvanic battery possesses the most surprising powers 
of chemical decomposition. 

399. The galvanic fluid or influence is excited by the 
contact of pieces of different metal, and sometimes by 
different pieces of the same metal. 

Illustration first. If a living frog, or a fish, (as a flounder,) 
having a slip of tin foil pasted on its back, be placed upon a piece 

* Dr. Aloysius Galvani was a Professor of Anatomy in Bologna, and made his 
discoveries about the year 1790. His wife, being consumptive, was advised to 
take, as a nutritive article of diet, some soup made of the flesh of frogs. Several 
of these animals, recently skinned for that purpose, were lying on a table in his 
laboratory, near an electrical machine, with which a pupil of the professor was 
amusing himself, in trying experiments, While the machine was in action he 
chanced to touch the bare nerve of the leg of one of the frogs, with the blade of a 
knife that he held in his hand, when suddenly the whole limb was thrown into vi- 
olent convulsions. Galvani being informed of the fact, repeated the experiment, 
and examined minutely all the circumstances connected with it. In this way he 
was led to the discovery of the principles which form the basis of this science. 
The science was subsequently extended by the discoveries of Professor Volta, of 
Pavia, who first constructed the Galvanic, or Voltaic Pile, in the beginning of the 
present century. 

396. What is galvanism? How is electricity generally produced? Fy whom 
and when was galvanism discovered? What led to the discovery? 397. How 
does the motion of the electric fluid, excited by galvanic power, differ from that 
explained in the science of electricity r 398. What bodies are most easily affect- 
ed by the galvanic fluid ? 399. How h the galvanic fluid or influence excited ? 
What illustrations of this are given I 



GALVANISM. 169 

of zinc, spasms of the muscles will be excited whenever a com- 
munication is made between the zinc and the tin foil. 

Illustration second. If a person place a piece of one metal, as 
a half dollar, above his tongue, and a piece of some other metal, 
as zinc, below the tongue, he will perceive a peculiar taste ; and, 
in the dark, will see a flash of light, whenever the outer edges of 
the metals are in contact. 

Illustration third. A faint flash may be made to appear before 
the eyes by putting a slip of tin foil upon the bulb of one of the 
eyes, a piece of silver in the mouth, and making a communication 
between them. In these experiments, no effect is produced so long 
as the metals are kept apart ; but on bringing them into contact, 
the effects above described are produced. 

400. The conductors of the galvanic fluid are divid- 
ed into the perfect and the imperfect. Metallic sub- 
stances, plumbago and charcoal, the mineral acids and 
saline solutions are perfect conductors. Water, oxyda- 
ted fluids, as the acids and all the substances that con- 
tain these fluids, alcohol, ether, sulphur, oils, resins, and 
metallic oxides, are imperfect conductors. 

401. To produce any galvanic action it is necessary 
to form what is called a galvanic circle ; that is, a cer- 
tain order or succession of substances capable of pro- 
ducing the fluid. 

402. To produce electricity in the common way (as has 
been stated wider the head of electricity,) it is necessary 
to excite an electric or non-conducting substance. But 
to produce the galvanic fluid, all that is necessary is the 
simple contact of different conducting substances with 
each other. 

403. The simplest galvanic circle is composed of 
three conductors, one of which must be solid, the oth- 
er fluid ; the third may be either solid or fluid. 

404. The process usually adopted for obtaining gal- 
vanic electricity is to place between two plates, of dif- 
ferent kinds of metals, a fluid capable of exerting some 
chemical action on one of the plates while it has no ac- 
tion, or a different action, on the other, A communi- 
cation is then formed between the two plates. 

400. Into what are the conductors of the galvanic fluid divided? What sub- 
stances are perfect conductors ? What substances are imperfect conductors? 
401. What is necessary in order to produce any gnlvanic action ? 402. How does 
the manner of producing the galvanic fluid and electricity in the common way, 
differ? 403. Of what is the simplest galvanic circle composed? 404. What pro- 
cess is usually adopted for obtaining galvanic electricity? 



170 



NATURAL PHILOSOPHY. 



Illustration. Fig. 121 
Fig. 127. 




represents a simple galvanic circle. It 
consists of a vessel containing a portion 
of diluted sulphuric acid, with a plate 
of zinc Z and of copper C immersed 
in it. The plates are separated at the 
bottom, and the circle is completed by 
uniting the plates at the top. The same 
effect will be produced, if, instead of 
allowing the metallic plates to come into 
direct contact, the communication be- 
tween them be effected by wires extend- 
ing from one to the other. 

In the above arrangement, there are 

If \|| three elements or essential parts :* name- 

I ptSpfifflBf 1 ly> ^e zinc, the copper, and the acid. 
|H m|| \y The acid, acting chemicallyt upon the 
zinc, produces an alteration in the elec- 
trical state of the metal. The zinc com- 
municating its natural share of the electri- 
cal fluid to the acid, becomes negatively t 
electrified. The copper, attracting the same fluid from the acid, be- 
comes positively electrified. Any conducting substance, therefore, 
placed M T ithin the line of communication between the positive and 
negative points, will receive the charge thus to be obtained. The 
arrows in fig. 127 show the direction of the current of positive 
electricity, namely, from the zinc to the fluid, — from the fluid to 
the copper, — from the copper back to the zinc. The substance to 
be submitted to the action of the fluid, must be placed in the line 
of communication between the copper and the zinc. 

* It is essential in all cases to have three elements to produce galvanic action. 
In the experiments or illustrations under No. 399, the moisture of the animal, or 
of the mouth, supplies the place of the acid, so that the three constituent parts of 
the circle are completed. 

t A certain quantity of electricity is always developed, or, in other words, con- 
verted from a latent to an active state, whenever a chemical action takes place 
between a fluid and a solid body. This is a general law of chemical action ; and, 
indeed, it has been ascertained that there is so intimate a connexion between elec- 
trical and chemical charges, that the chemical action can, proceed only to a cer- 
tain extent, unless the electrical equilibrium, which has been disturbed, be again 
restored. Hence, we find that in the simple, as well as in the compound galvanic 
circle, the oxidation of the zinc proceeds with activity whenever the galvanic cir- 
cle is completed ; and that it ceases, or, at least, takes place very slowly, whenev- 
er the circuit is interrupted. 

% It is a singular fact that in a simple galvanic circle, composed of zinc, acid 
and copper, the zinc end will always be negative, and the copper end positive; but 
in all compound galvanic circles, composed of the same elements, the zinc will be 
positive, and the copper negative. 



Illustrate this by fig. 127. What effect will be produced if, instead of allowing 
the metallic plates to come into direct contact, the communication between thein 
be effected by wires > How many parts are there in the above arrangement? 
What are they ? What effect does the acid produce.' What is the electrical 
gtate of the zinc ? Of the copper? What singular fact is related in the note? 
What are the arrows, in fig. 127, designed to show .' Where must the substanco > 
to be submitted to the action of the fluid, be placed? 



GALVANISM. 171 

The electrical effects of a simple galvanic circle, such as has 
now been described, are, in general, too feeble to be perceived, 
except by very delicate tests. The muscles of animals, especially 
those of cold-blooded animals, such as frogs, &c, the tongue, the 
eye, and other sensitive parts of the body, being very easily affected, 
afford examples of the operation of simple galvanic circles. (See 
Illustrations under No. 399.) In these, although the quantity of 
electricity set in motion is exceedingly small, it is yet sufficient to 
produce very considerable effects ; but it produces little or no effect 
on the most delicate electrometer.* 

405. The galvanic effects of a simple circle may be 
increased, to any degree, by a repetition of the same 
simple combination. Such repetitions constitute com- 
pound galvanic circles, and are called galvanic piles 
or galvanic batteries, according to the mode in which 
they are constructed. 

406. The voltaic pile consists of alternate plates of 
two different kinds of metal, separated by woollen 
cloth, card, or some similar substance. 

Illustration. Fig. 128 represents a voltaic pile. A voltaic pile 
may be constructed in the following manner : Take a number, say 
twelve plates of silver, and the same num- 
ber of zinc, and also of woollen cloth, the 
cloth having been soaked in a solution of 
sal ammoniac in water ; with these a pile 
is to be formed in the following order : 
namely, a piece of silver, a piece of zinc, 
a piece of cloth, and thus repeated. These 
are to be supported by three glass rods, 
placed perpendicularly with pieces of wood 
at the top and bottom, and the pile will 
then be complete ; and will afford a con- 
stant current of electric fluid through any conducting substance. 
Thus, if one hand be applied to the lower plate, and the other to 
the upper one, a shock will be felt, which will be repeated as often 
as the contact is renewed. . 

Instead of silver, copper plates, or plates of other metal, may be 
used in the above arrangement. The arrows in the figure, show 
the course of the current of electricity in the arrangement of sil- 
ver, zinc, &c. 

* On the principle of the simple galvanic circle, Dr. Hare, of Philadelphia, 
constructed a very powerful apparatus, which he called a Calorimotor, from 
its remarkable property of producing heat. 

What is said of the electrical effects of a simple galvanic circle? What exam- 
ples are given illustrating the operation of simple galvanic circles ? Upon what 
principle is the calorimotor constructed ? 405. How may the galvanic effects of 
the simple circle be increased ? What are compound galvanic circles? 406. Of 
what does the voltaic pile consist ? What does fig. 128 represent ? How may a 
voltaic pile be constructed > Can any other metal be used ? What are the arrows 
in the figure designed to show I 




172 



NATURAL PHILOSOPHY. 



Fig. 129. 



wmMmmmt? 




407. The voltaic battery is a combination of metal- 
lic plates, immersed by pairs in a fluid which exerts a 
chemical action on one of each pair of the plates, and 
no action, or, at least, a different action on the other.* 

Illustration. Fig. 129 represents a Voltaic battery. It consists of 
a trough made of baked wood, wedgewood-ware, or some other 
non-conducting substance. It is divi- 
ded into grooves or partitions, for the 
reception of the acid, or a saline solu- 
tion, and the plates of zinc or copper 
(or other metals) are immersed by 
pairs in the grooves. These pairs of 
. . . , plates are united by a slip of metat 
^liSO^ilr'ili ,'iX ii! ^ :; jSif passing from the one and soldered to 
' ! H the other; each pair being placed so 

as to enclose a partition between them, 
and each cell or groove in the trough 
containing a plate of zinc, connected 
with the copper plate of the succeeding cell, and a copper plate 
joined wth the zinc plate of the preceding cell. These pairs must 
commence with copper and terminate with zinc, or commence with 
zinc and terminate with copper. The communication between the 
first and last plates is made by wires, which thusg complete the 
galvanic circuit. The substance to be submitted to galvanic action 
is placed between the points of the two wires. 

A compound battery of great power is obtained by uniting a 
number of these troughs. In a similar manner a battery may be 
produced by uniting several piles, making a metallic communi- 
cation between the last plate of the one and the first plate of the 
next, and so on, taking care that the order^ of succession of the 
plates in the circuit be preserved inviolate. 

The Couronne des tasses, represented in figure 130, is another 
form of the galvanic battery. It 
consists of a number of cups, 
bowls, or glasses, with the zinc 
and copper plates immersed in 
them, in the order represented in 
the figure ; Z indicating the zinc, 
and "C the copper plates; the 
arrows denoting the course of the 
electric fluid. 

* The electricity excited by the bat- 
tery, proceeds from the solid to the fluid 
which acts upon it chemically. Thus, 
in a battery- composed of zinc, diluted 
sulphuric acid and copper, the acid acts upon the zinc, and not on th»6opp«r. 
The galvanic fluid proceeds, therefore, from the ;zmc to the add, from the acid to 
the copper, &c. 

407. What is the voltaic battery ? What U said in the note with regard to the 
electricity excited by the battery ? . What does fig. 129 represent ? Of wba ; doe. 
the voltaic battery consist ? How is the communication between the first ana last 
plates made.' Where must the substance which is to be submitted to galyaiuc 
action be placed ? How can a compound battery of great power be obtained . 
What does fig. 130 represent.' Of what does this battery consist .' 



Fig. 130. 




GALVANISM. 173 

The electric shock from the voltaic battery may be received 
by any number of persons, by joining hands, having previously 
wetted them. 

The spark from a powerful voltaic battery acts upon and inflames 
gunpowder, charcoal, cotton, and other inflammable bodies, melts 
all metals, disperses diamonds, &c. 

The wires, by which the circuit of the battery is completed, are 
generally covered with glass tubes, in order that they may be held, 
or directed to any substance. 

408. There are three principal circumstances in 
which the electricity produced by the galvanic or vol- 
taic battery, differs from that obtained by the ordinary 
electrical machine, namely, — 

First. The very low degree of intensity* of that produced by 
the galvanic battery, compared with that obtained by the machine. 

Secondly. The very large quantity of electricity which is set in 
motion by the voltaic battery ; and 

Thirdly. The continuity of the current of voltaic electrici- 
ty, and its perpetual reproduction, even while this current is tend- 
ing to restore the equilibrium.! 

* By intensity is here meant the same that is implied by density, as applied to 
matter. The quantity of electricity obtained by galvanic action is much greater 
than can be obtained by the machine : but it flows, as it were, in narrow streams. 
The action of the electrical machine may be compared to a mighty torrent, dash- 
ing and exhausting itself in one leap from a precipitous height. The galvanic 
action may he compared to a steady stream, supplied by an inexhaustible fountain. 
In other words, the momentum of the electricity excited by galvanism is less than 
that from the electrical machine — but the quantity, as has* been stated, is greater. 

f Whenever an electrical battery is charged, how great soever may be the quan- 
tity that it contains, the whole of the power is at once expended, as soon as the 
circuit is completed. Its action may be sufficiently energetic while it lasts, but 
it is exerted only for an instant, and like the destructive operation of lightning, 
can effect, during its momentary passage, only sudden and violent changes, which 
it is beyond human power to regulate or control. On the contrary, the voltaic 
battery continues for an indefinite time, to develop and supply vast quantities of 
electricity, which, far from being lost by returning to their source, circulate in a 
perpetual stream, and with undiminished force. The effects of this continued cur- 
rent on the bodies subjected to its action, will, therefore, be more definite, and 
will be constantly accumulating; and their amount, in process of time, will be 
incomparably greater than even those of the ordinary electrical explasion. It is, 
therefore, found that changes in the composition of bodies are effected by galvan- 
ism, which can be accomplished by no other means. The science of galvanism, 
therefore, has extended the field, and muliiplied the means of investigation in the 
kindred sciences, especially that of chemistry. 

How can the electric shock, from the voltaic battery, he received by any num- 
ber of persons.' What is said of the spark from a powerful voltaic battery; 408. 
In how many ways does the electricity produced by the galvanic or voltaic battery 
differ from that obtained by the ordinary electrical machine! What is the first.' 
What is here meant by intensity ? How dr.es the quantity of electricity obtained 
by galvanic action, compare with that obtained by the machine? To what may 
the action of the electrical machine be compared ? To what may the galvanic ae- 
tion be compared ? What is the second way in which they differ ? What is the 
third; What is said in the note with regard to the third circumstance iD which 
the electricity obtained by the ordinary electrical machine differs from that pro- 
duced by the galvanic battery ? What is said of the effects of this continued 
current on the bodies subjected to its action ? 

16 



174 NATURAL PHILOSOPHY. 

A common electrical battery may be charged from a voltaic 
battery of sufficient size ; but the largest calorimotor that has yet 
been constructed, furnishes no indication of attraction or repulsion 
equal to that which is given by the feeblest degree of excitation to 
a piece of sealing wax. A galvanic battery, consisting of fifty 
pairs of plates, will affect a delicate gold-leaf electrometer ; and, 
with a series of one thousand pairs, even pith balls are made to 
diverge. 

Voltaic piles have been constructed of layers of gold and silver 
paper. The effect of such piles remains undisturbed for years. 
With the assistance of two such piles, a kind of perpetual motion, 
or self-moving clock, has been invented by an Italian philosopher. 
The motion is produced by the attraction and repulsion of the 
piles exerted on a pith ball, on the principle of the electrical bells 
described on page 161, No. 11. The top of one of the piles was 
positive, and the bottom negative. The other pile was in an oppo- 
site state ; namely, the top negative, and the bottom positive. 

409. The effect of the voltaic pile on the ani- 
mal body depends chiefly on the number of plates 
that are employed ; but the intensity of the spark 
and its chemical agencies increase more with the size 
of the plates, than with their number. 

410. Galvanism explains many facts in common life. 
Porter, ale, or strong beer, is said to have a peculiar taste when 

drunk from a pewter vessel. The peculiarity of taste is caused 
by the galvanic circle formed by the pewter, the beer, &c, and the 
moisture of the under lip. 

Silver is tarnished by the yolk of an egg. Here is another gal- 
vanic circle formed by the yolk, the silver, and the moisture of the 
tongue. 

Works of metals, the parts of which, are soldered together, soon 
tarnish in the places where the metals are joined. 

Ancient coins, composed of a mixture of metal, have crumbled to 
pieces, while those composed of pure metal have been uninjured. 

The nails and the copper in sheathing of ships are soon corroded 
about the place of contact. These are all the effects of galvan- 
ism.* 

* The most striking effects of galvanism on the human frame, after death, were 
exhibited at Glasgow, a few years ago. The subject on which the experiments 
were made was the body of the murderer Clydesdale, who was hanged at that 
city. He had been suspended an hour, and the first experiment was made in about 
ten minutes after he was cut down. The galvanic battery employed consisted of 
270 pairs of four inch plates. On the application of the battery to different parts 
of the body, every muscle was thrown into violent agitation ; the leg was thrown 
out with great violence, breathing commenced, the face exhibited extraordinary 
grimaces, and the finger seemed to point out the spectators. Many persons were 
obliged to leave the room from terror or sickness; one gentleman fainted, and some 
thought that the body had really come to life. 

Of what have voltaic piles been constructed ? What has been produced with 
the assistance of two such piles ? How and on what principle was the motion 
produced .' 409. On what does the effect of the voltaic pile on the body depend ? 
410. What facts in common life does galvanism explain? 



MAGNETISM. 175 

There are persons who profess to be able to find out seams in brass 
and copper vessels by the tongue, which the eye cannot discover ; 
and, by the same means, to distinguish the base mixtures which 
abound in gold and silver trinkets. 



SECTION XVIII. 

Magnetism. 

411. Magnetism treats of the properties and effects 
of the magnet, or loadstone. 

412. There are two kinds of magnets, namely, the 
native or natural magnet, and the artificial. 

413. The native magnet, or loadstone, is an ore of 
iron, found in iron mines, and has the property of at- 
tracting iron and other substances which contain it. 

414. An artificial magnet is a piece of iron to which 
magnetic properties have been communicated. 

For all purposes of accurate experiment, the artificial is to be pre- 
ferred to the native magnet. 

415. If a straight bar of hard tempered steel be held 
in a vertical position, (or, still better, in a position 
slightly inclined to the perpendicular, the lower end de- 
viating to the north,) and struck several smart blows 
with a hammer, it will be found to have acquired, by 
this process, all the properties of a magnet ; or, in other 
words, it will become an artificial magnet. 

416. The properties of a magnet are four ; namely, 
First, Polarity — Second, Attraction of unmagnetic iron 
— Third, Attraction and repulsion of magnetic iron — 
Fourth, The power of communicating magnetism to 
other iron. 

417. By the polarity of a magnet is meant the prop- 
erty of pointing, or turning to the north and south poles. 



411. Of what does magnetism treat ? 412. How many kinds of magnets are there I 
What are they >. 413. What is the native magnet ! What property does it pos- 
sess? 414. What is an artificial magnet? Which magnet is preferred, for all 
purposes of accurate experiment ? 415. How can an artificial magnet be made ? 
416. What is the first property of the magnet? Second ? Third I Fourth? 417. 
What is meant by the polarity of a magnet 2 



176 NATURAL PHILOSOPHY. 

The end which points to the north, is called the north 
pole of the magnet, and the other the south pole. The 
attractive power of a magnet is strongest at the poles. 

When a magnet is supported in such a manner as to move 
freely, it will spontaneously assume a position directed nearly 
north and south. The end which points to the north, is called the 
north pole of the magnet ; and that which points to the south, is 
called the south pole. 

There are several ways of supporting a magnet, so as to enable 
it to manifest its polarity. First, by suspending it, accurately 
balanced, from a string. Secondly, by poising it on a sharp point. 
Thirdly, by fixing it on a piece of cork, and thus making it float 
on water. 

418. A magnet, whether native or artificial, attracts 
iron or steel which has no magnetic properties; but it 
both attracts and repels those substances, when they are 
magnetic ; that is, the north pole of one magnet will at- 
tract the south pole of another, and the south pole of 
one will attract the north of another ; but the north pole 
of the one repels the north pole of the other, and the 
south pole of one repels the south pole of another. In 
few words, different poles attract, and similar poles re- 
pel each other.* 

If either pole of a magnet be brought near any small piece of 
soft iron, it will attract it. Iron filings will also adhere in clusters 
to either pole. 

If the north pole of a magnet, held in the hand, be presented to 
the same pole of a magnet balanced on a point, or suspended by a 
string, it will repel it — but it will attract the opposite pole. 

419. A magnet may communicate its properties to 
other bodies. But these properties can be conveyed to 
no other substances than iron, nickel or cobalt. t All 

* There is here a close analogy between the attractive and repulsive powers of 
the different kinds of electricity, (that is, the positive and the negative,) and the 
northern and southern polarities of the magnet. The same law obtains with re- 
gard to both; namely, — between like powers there is repulsion; between unlike, 
there is attraction. 

fThe accuracy of the above statement may, perhaps, be questioned, since Cou- 
lomb has discovered that : ' all solid bodies are susceptible of magnetic influence." 
But the " influence " is perceptible only by the nicest tests, and under peculiar cir- 
cumstances. [Sec Electro-Magnetism.] 

Where is the attractive power of the magnet the strongest? When will a 
magnet assume a position directed nearly north or south? What is the north 
pole of the magnet? What is the south pole? In what ways can a magnet be 
supported so as to enable it to manifest its polarity. 418. What is said in No. 418 
with regard to the attraction of magnets, whether native or artificial .' What 
analogv is there between the attractive and repulsive powers of the different kinds 
of electricity, and the northern and southern polarities of the magnet? 419. Can 
a magnet communicate its properties to other bodies ? To what substances, only, 
can these properties be conveyed > 



MAGNETISM. 177 

natural and artificial magnets, as well as the bodies 
on which they act, are either iron in its pure state, or 
such compounds as contain it. 

420. The powers of a magnet are increased by ac- 
tion, and are impaired and even lost by long disuse. 

When the two poles of a magnet are brought together, so that 
the magnet resembles in shape a horse-shoe, it is called a horse- 
shoe magnet, and it may be made to sustain a considerable weight 
by suspending substances from a small iron bar, extending from 
one pole to the other. This bar is called the keeper. A small 
addition may be made to the weight every day. 

421. Soft iron acquires the magnetic power very 
readily, and also loses it as readily — but hardened iron 
or steel acquires the property with difficulty, but when 
it has acquired it, retains it permanently. 

422. When a magnet is broken or divided, each part 
becomes a perfect magnet, having both a north and 
south pole. 

This is a remarkable circumstance, since the central part of a 
magnet appears to possess but little of the magnetic power — but 
when a magnet is divided in the centre, this very part assumes the 
magnetic power, and becomes possessed, in the one part, of the 
north, and in the other, of the south polarity. 

423. The magnetic power of iron or steel resides 
wholly on the surface, and is independent of its mass.* 

* In this respect there is a strong resemblance between magnetism and electrici- 
ty. Electricity, as has already been stated, is wholly confined to the surface of 
bodies. In a few words, magnetism and electricity may be said to resemble each 
other in the following particulars. 

1. Each consists of two species, namely, the vitreous and the resinous (or, the 
positive and negative) electricities ; and the northern and southern (sometimes 
called the Boreal, and the Jlustral) polarity. 

2. In both magnetism and electricity, those of the same name repel and those 
of different names attract each other [See JVb. 418.] 

3. The laws of induction in both are similar. 

4. The powers of attraction and repulsion in each vary inversely as the square 
of the distance. [See JVb. 74, page 20.] 

5. The influence, in both cases, (as has just been stated,) resides at the surface, 
and is wholly independent of their mass. 

But magnetism and electricity differ in the following particulars. 

1. Electricity is capable of being excited in all bodies, and of being imparted 

Of what substance are all natural and artificial magnets, as well as the bodies 
on which they act, composed ? 420. How can the powers of a magnet be increas- 
ed ! What is a horse-shoe magnet >. How can it be made to sustain a considerable 
weight! What is this bar called? 421. How does soft iron differ from hardened 
iron, with respect to its acquiring and losing the magnetic power ? 422. Does the 
breaking of a magnet cause any loss of its magnetic power.' Why is this a re- 
markable circumstance ? 423. Where does the magnetic power of iron or steel 
wholly reside ! In what particulars do magnetism and electricity resemble each 
other? 1. What is the first! 2. What is the second! 3. What is the third? 
4. What is the fourth ? 5. What is the fifth ! In what particulars do magnetism 
and electricity differ from each other? 1. What is the first ? 

16* 



178 NATURAL PHILOSOPHY. 

424. Heat weakens, and a great degree of heat de- 
stroys the power of a magnet ; but the magnetic attrac- 
tion is not in the least diminished by the interposition 
of any bodies except iron, steel, &c. 

Electricity frequently changes the poles of a magnet ; and the 
explosion of a small quantity of gunpowder, on one of the poles, 
produces the same effect. 

Electricity, also, sometimes renders iron and steel magnetic, 
which were not so before the charge was received. 

425. The effects produced by two magnets, used 
together, are much more than double that of either one 
used alone. 

426. When a magnet is suspended freely from its 
centre, the two poles will not lie in the same horizontal 
direction ; one of them will incline towards the hori- 
zon, and the other will consequently rise ; or, in other 
words, one end of the magnet will be higher than the 
other. This is called the inclination or the clipping of 
the magnet. 

427. The magnet, when suspended, does not invaria- 
bly point exactly to the north and south points, but va- 
ries a little towards the east or the west. This varia- 
tion differs at different places, at different seasons, and 
at different times in the day- 

428. The science of magnetism has rendered im- 
mense advantages to commerce and navigation, by 

to all — Magnetism, with but few exceptions, reside? in, and can be imparted only 
o iron, and its <liffe-rent compounds. [See note to JVe. 419.] 

2. Electricity may be transferred from one body to another ; in which case the 
body from which it is transferred loses the whole or a portion of its elechicity. 
Magnetism cannot he transfeired in the same manner ; but it may be communicat- 
ed from a magnet to another piece of iron or steel, in which case the magnet em- 
ployed loses no part, of its own power. 

3. When an electrified body is divided near the middle, the two parts will pos- 
sess the same kind of electricity which they had before the separation — but when 
a magnet is divided, or broken into any number of parts, each part will have both 
polarities, and become a perfect magnet. 

4. The directive property, or the property of turning toward the noith and 
south poles, belongs to the magnet alone. 

2. What is the second? 3. What i3 the third ? 4. What is the fourth? 
424. What effecr, has heat on the power of a magnet ? By what is the magnetic 
attraction diminished ; What effect has electricity on the poles of a magnet? 
What effect has electricity, sometimes, on iron and steel? 425. What proportion 
do the effects produced bv two magnets, used together, hear to that of either used 
alone? 426. What is meant by the inclination or dipping of the magnet ? 427. 
Does the magnet, when suspended, invariably point exactly to the north and south 
points? 428. What immense advantage has the seience of magnetism rendered 
to commerce and navigation? 



MAGNETISM. 179 

means of the mariners' compass.* The mariner's 
compass consists of a magnetised bar of steel, call- 
ed a needle; having at its centre a cap fitted to it, 
which is supported on a sharp-pointed pivot fixed in 
the base of the instrument. A circular plate, or card, 
the circumference of which is divided into degrees, is 
attached to the needle, and turns with it. On an inner 
circle of the card the thirty-two points of the mariner's 
compass are inscribed. t 

The needle is generally placed under the card of a mariner's 
compass, so that it is out of sight ; but small needles, used on land, 
are placed above ihe card, and the card is permanently fixed to 
the box. 

429. The north pole of a magnet is more powerful 
in the northern hemisphere, or north of the equator, 
and the south pole in the southern parts of the world. 

430. When apiece of iron is brought sufficiently near 
to a magnet, it becomes itself a magnet ; and bars of 
iron, that have stood long in a perpendicular situation, 
are generally found to be magnetical. 

Artificial magnets are made by applying one or more power- 
ful magnets to pieces of hard steel. The end which is touched 
by the^north pole becomes the south pole of the new magnet, 
and that touched by the south pole, becomes the north pole. 
The magnet which is employed in magnetising a steel bar loses 
none of its power by being thus employed ; and as the effect is 

* The invention of the mariner's compass is usually ascribed to Flavio de Melfi, 
or Flavio Gioia, a Neapolitan, about the year 1302. Some authorities, howevm, 
assert ihat it was brought from China, by Marcus Paulus, a Venetian, in J260. 
The invention is also claimed both by the French and English. 

The value of this discovery may be estimated from the consideration, that, untit 
then, mariners seldom trusted themselves out of sight of land ; they were unable 
to make long or distant voyages, as they had no means to find their way back. 
This discovery enabled them to find a way where all is trackless — to conduct their 
vessels through the mighty ocean, out of the sight of land ; and to prosecute 
those discoveries, and perform those gallant deeds which have immortalized the 
names of Cook, of La Perouse, Vancouver, Sir Francis Drake, Nelson, Parry, 
Franklin, and others. 

f The compass is generally fitted by two sets of axes to an outer box, so that it 
always retains a horizontal position, even when the vessel rolls. When the arti- 
ficial magnet or needle is kept thus freely suspended, so that it may turn North 
or South, the pilot, by looking at its position, can ascertain in what direction his 
vessel is proceeding ; (See No. 417,) and, although the needle varies a little from 
a correct polarity, yet this variation is never so great, or so irregular, as seriously 
to impair its use as a guide to the vessel in its course over the pathless deep. 

Of what does the mariner's compass consist? To whom is the inventioaof the 
mariner's compass usually ascribed.' How may the value of this discovery be 
estimated .' 429 Where are the north and south poles of a magnet the most 
powerful ? 430. What effect has a magnet on a piece of iron, when it is brought 
sufficiently near to it >. How are artificial magnets made ? Does the magnet, which 
is employed in magnetising a steel bar, lose any of its power by being thus em- 
ployed ? 



180 NATURAL PHILOSOPHY. 

increased when two or more magnets are used, with one magnet a 
number of bars may be magnetised, and then combined together ; 
by which means their power may be indefinitely increased. Such 
an apparatus is called a magnetic magazine* 

A magnetic needle is made by fastening the steel on a piece of 
board and drawing magnets over it from the centre outwards. 

A horse-shoe magnet should be kept armed, by a small piece of 
iron or steel, connecting the two poles. 

Interesting experiments may be made by a magnet, even of no 
great power, with steel or iron filings, small needles, pieces of ferru- 
ginous substances, and black sand, which contains iron. Such sub- 
stances may be made to assume a variety of amusing forms and po- 
sitions, by moving the magnet under the card, paper, or table, on 
which they are placed. Toys, representing fishes, frogs, &c, which 
are made to appear to bite at a hook, birds, floating on the water, &c. 
are constructed on magnetic principles, and sold in the shops. 



SECTION XIX. 

Electro-Magnetism. 

431. Electro-Magnetism treats of the combined pow- 
ers of electricity and magnetism. 

* There are many methods of making artificial magnets. One of the most sim- 
ple and effectual consists in passing a strong horse-shoe magnet over bars previous- 
ly hardened and prepared. 

In making bar (or straight) magnets, the bars must be laid lengthwise, on a flat 
table, with the marked end of one bar against the unmarked end of the next ; and 
in making horse-shoe magnets, the pieces of steel previously bent into their proper 
form, must be laid with their ends in contact, so as to form a figure like two capi- 
tal LL'sj with their tops joined together, thus Cj£? > observing that the marked 
ends come opposite to those which are not marked ; and then, in either case, a 
strong horse-shoe magnet is to be passed with moderate pressure over the bars ; 
taking care to let the marked end of this magnet precede, and its unmarked end 
follow it ; and to move it constantly over the steel bars, so as to enter or commence 
the process at a mark, and then to proceed to an unmarked end, and enter the 
next bar at its marked end, and so proceed. 

After having thus passed over the bars ten or a dozen times on each side, and 
in the same direction, as to the marks, they will be converted into tolerably strong 
and permanent magnets. But if, after having continued the process for some 
time, the exciting magnet is moved even onceover the bars in a contrary direction, 
or if its south pole should be permitted to precede, after the north pole has been 
first used, all the previously excited magnetism will disappear, and the bars will 
be found in their original state. 

What is a magnetic magazine? How is a magnetic needle made? What is 
said with regard to a horse-shoe magnet ? 431. Of what does electro-magnetism 
treat ? 



ELECTRO-MAGNETISM. 181 

432. The passage of the two kinds of electricity, 
(namely, the positive and the negative,) through their 
circuit, is called the electric currents ; and the science 
of electro-magnetism explains the phenomena attend- 
ing those currents. 

It has already been stated, that from the connecting wires of the 
galvanic circle, or battery, there is a constant current of electricity 
passing from the zinc to the copper, and from the copper to the 
zinc plates. In the single circle these currents will be negative from 
the zinc, and positive from the copper ; but in the compound circles, 
or the battery, the current of positive electricity will flow from the 
zinc to the copper, and the current of negative electricity from the 
copper to the zinc. 

433. From the effect produced hy electricity, on the 
magnetic needle, it had been conjectured, by a number 
of eminent philosophers, that magnetism, or magnetic 
attraction is in some manner caused by electricity. In 
the year 1819, Professor Oersted, of Copenhagen, 
made the grand discovery of the power of the electric 
current to induce magnetism ; thus proving the connex- 
ion between magnetism and electricity. 

434. In a short time after the discovery of Professor 
Oersted, Mr. Farraday discovered that an electrical 
spark could be taken from a magnet ; and thus the com- 
mon source of magnetism and electricity was fully 
proved. 

In a paper recently published, this distinguished philosopher has 
very ably maintained the identity of common electricity, voltaic 
electricity, magnetic electricity, (or electro-magnetism,) thermo- 
electricity,* and animal electricity. The phenomena exhibited in 
all these five kinds of electricity, differ merely in degree and the 
state of intensity in the action of the fluid. 

* In the year 1822, Professor Seebeck, of Berlin, discovered that currents of elec- 
tricity might be produced by the partial application of heat to a circuit composed 
exclusively of solid conductors. (See Ga/vanisvi, No. 403.) The electrical cur- 
rent, thus excited, has been termed Thermoelectric, (from the Greek Thermos, 
which signifies heat,) to distinguish it from the common galvanic current ; which, 
as it requires the intervention of a fluid element, was denominated a Hydroelec- 
tric current. The term Stereo -eleelric current has also been applied to the former, 
in order to mark its being produced in systems formed of solid bodes alone. It is- 

432. What is the electric current ! What does the science of electro-magnetism 
explain I What is the difference between the currents in the single and the com- 
pound circles ! 433. What is it thought causes magnetic attraction ? What dis- 
covery was made in the year 1819! By whom ? 4'.!47 What farther discovery was 
made soon after, and by whom ? What does this philosopher maintain ? How do- 
the phenomena exhibited in these five kinds of electricity differ >. In what way 
may currents of electricity be produced ? What is the electrical current, thus ex- 
cited, termed? How does this current differ from the common galvanic current ? 
What other term has been applied to this current ? 



182 NATURAL PHILOSOPHY. 

The discovery of Professor Oersted has been followed out by Am- 
pere ; who, by his mathematical and experimental researches, has 
presented a theory of the science less obnoxious to objections than 
that proposed by the Professor. 

435. The principal facts in connexion with the sci- 
ence of electro-magnetism are, — 

1. That the electric current, passing uninterruptedly * through a 
wire, connecting the two ends of a galvanic battery, produces an 
effect upon the magnetic needle. 

2. That electricity w r ill induce magnetism. 

3. That a magnet, or a magnetic magazine, will induce electricity. 

4. That the combined action of electricity and magnetism, as de- 
scribed in the science, produces a rotatory motion of certain kinds 
of bodies, in a direction pointed out by certain laws. 

5. That the periodical variation of the magnetic needle, from 
the true meridian, or, in other words, the variation of the compass 
is caused by the influence of the electric currents. 

6. That the magnetic influence is not confined to iron, steel, &c. 
(See Magnetism, No. 419,) but that most metals, and many other 
substances may be converted into temporary magnets by electrical 
action. 

7. That the magnetic attraction of iron, steel, &c. may be prodi- 
giously increased by electrical agency. 

8. That the direction of the electric current may, in all cases, be 
ascertained.! 

evident that if, as is supposed in the theory of Ampere, magnets owe their pecu- 
liar properties to the continual circulation of electric currents in their minute 
parts, these currents will come under the description of the stereo-electric currents. 
From the views of electricity which have now been given, it appears that there 
are, strictly speaking, three states of electricity. That derived from the common 
electrical machine is in the highest degree of tension, and accumulates until it is 
able to force its way through the air, which is a perfect nonconductor. In the 
galvanic apparatus, the currents have a smaller degree of tension ; because, al- 
though they pass freely through the metallic elements, they meet with some im- 
pediments in traversing the fluid conductor. But in the thermo-electric currents, 
the tension is reduced to nothing ; because, throughout the whole course of the 
circuit, no impediment exists to its free and uniform circulation. 

* All the effects of electricity and galvanism, that have hitherto been described 
have been produced on bodies interposed between the extremities of conductors, 
proceeding from the positive and negative poles. It was not known, until the dis- 
coveries of Professor Oersted were made, that any effect could be produced when 
the electric circuit is uninterrupted. 

\ This i3 done by means of the magnetic needle. If a sheet of paper be placed 
over a horse-shoe magnet, and fine black sand, or steel filings, be dropped loosely 
on the paper, the particles will be disposed to arrange themselves in a regular or- 
der, and in the direction of curve lines. This is, undoubtedly, the effect of some 
influence, whether that of electricity, or of magnetism alone, cannot at present 
be determined. 

To what do magnets owe their peculiar properties? What follows from this? 
How many states of electricity are there J What is said of that derived from 
the common electrical machine.' What is said of that derived from the galvanic 
apparatus? What is said of the thermoelectric currents? 435. What are the 
principal facts in connexion with the science of electro magnetism? 1. What is 
the first ? 2. What is the second ? 3. What is the third .' 4. What is the fourth i 
5. What is the fifth ? 6. What is the sixth ? 7. What is the seventh ? 8 What 
is the eighth ? Where have the bodies been placed, in all the effects of electricity 
and galvanism that have hitherto been described .' 



ELECTRO-MAGNETISM. 183 

9. That magnetism is produced whenever concentrated electricity- 
is passed through space. 

10. That while in common electrical and magnetic attractions 
and repulsions, those of the same name are mutually repulsive, and 
those of different names attract each other ; in the attractions and re- 
pulsions of electric currents, it is precisely the reverse, the repul- 
sion taking place only when the wires are so situated that the cur- 
rents are in opposite direction. 

436. A metallic wire, forming a part of a voltaic cir- 
cuit, exercises a peculiar attraction towards a magnet- 
ic needle. 

Illustration. If a wire, which connects the extremities of a vol- 
taic battery, be brought over, and parallel with a magnetic needle 
at rest, or with its poles properly directed north and south, that end 
of the needle next to the negative pole of the battery will move to- 
wards the west, whether the wire be on one side of the needle or the 
other, provided, only, that it be parallel with it. 

Again : If the connecting wire be lowered on either side of the 
needle, so as to be in the horizontal plane in which the needle should 
move, it will not move in that plane, but will have a tendency to 
revolve in a vertical direction ; in which, however, it will be pre- 
vented from moving, in consequence of the attraction of the earth, 
and the manner in which it is suspended. When the wire is to 
the east of the needle, the pole nearest to the negative extremity of 
the battery will be elevated ; and when it is on the west side, that 
pole will be depressed. 

If the connecting wire be placed below the plane in which the 
needle moves, and parallel with it, the pole of the needle next to 
the negative end of the wire will move towards the east ; and the 
attractions and repulsions will be the reverse of those observed in 
the former case. 

437. The two sides of an unmagnetised steel needle 
will become endued with the north and south polarity, 
if the needle be placed parallel with the connecting 
wire of a voltaic battery, and nearly or quite in con- 
tact with it. But if the needle be placed at right an- 
gles with the connecting wire, it will become perma- 
nently magnetic ; one of its extremities pointing to the 
north pole and the other to the south, where it is freely 
suspended and suffered to vibrate undisturbed. 

9. What is the ninth fact in connexion with the science of electro-magnetism .' 
10. What is the tenth ? How can the direction of the electric current be ascertain- 
ed .' 436. What is stated in No. 436 >. What illustration of this is given ? What 
second illustration is given i Where will the pole of the needle next to the negative 
end of the wire move~if the connecting wire be placed below the plane in which 
the needle moves, and parallel with it > What is said with regard to the attrac- 
tions and repulsions? 437. How may the two sides of an unmagnetised steel 
needle become endued with the north and south polarity? When will it become 
permanently magnetic? 



184 NATURAL PHILOSOPHY. 

438. Magnetism may be communicated to iron and 
steel by means of electricity from an electrical ma- 
chine ; but the effect can be more conveniently pro- 
duced by means of the voltaic battery. 

Illustration. If a helix be formed of wire, and a bar of steel be 
inclosed within the helix, the bar will immediately become magnet- 
ic by applying the conducting wires of the battery to the extremities 
of the helix * The electricity from the commonelectrical machine, 
when passed through the helix, will produce the same effect. 

If such a helix be so placed that it may move freely, as when 
made to float on a basin of water, it will be attracted and repelled 
by the opposite poles of a common magnet. 

439. If a magnetic needle be surrounded by coiled 
wire, covered with silk, a very minute portion of elec- 
tricity through the wire will cause the needle to deviate 
from its proper direction. 

A needle thus prepared, is called an electro-magnetic multiplier. 
It is in fact a very delicate electroscope, or rather galvanometer — 
capable of pointing out the direction of the electric current in all 
cases. 

The discovery of Oersted was limited to the action of the electric 
current on needles previously magnetised ; it was afterwards as- 
certained by Sir Humphrey Dav}\ and M. Arago, that magnetism 
may be developed in steel, not previously possessing it, if the steel 
be placed in the electric current. Both of these philosophers, inde- 
pendently of each other, ascertained that the uniting wire, becom- 
ing a magnet, attracts iron filings, and collects sufficient to acquire 
the diameter of a common quill; but the moment the connexion is 
broken, all the filings drop off; and the attraction diminishes with 
the decaying energy of the pile. Filings of brass, or copper, or 
wood shavings, are not attracted at all. 

440. Among the most remarkable of the facts con- 
nected with the science of electro-magnetism, is what 
is called the electro-magnetic rotation. Any wire, 
through which a current of electricity is passing, has a 

* The helix \a a spiral line, or a line in the form of a cork screw. The wire 
which forms the helix should be coated with some non-conducting substance, such 
as silk wound round it ; as it may then be formed into close coils, without suffering 
the electric fluids to pass from surface to surface, which would impair its effect. 

438. How can magnetism be communicated to iron and steel I How can the ef- 
fect be more conveniently produced ! What illustration of this is given.' What 
is the helix ? Why should the wire, which forms the helix be eoated with some 
non-conducting substance ? What is said of a helix, if it be placed so that it may 
move free iy r 439. How can the magnetic needle be made to deviate from its 
proper direction r What is a needle thus prepared called > Can magnetism be 
developed in steel not previously possessing it ? Where must the steel be placed ? 
What property has the uniting wire ? W T hat follows, if the connexion be brok- 
en? Are filings of brass or wood attracted at all? 440. What is the electro-mag- 
netic rotation ! 



ELECTRO-MAGNETISM. 



185 



tendency to revolve round a magnetic pole, in a plane 
perpendicular to the current ; and that without refer- 
ence to the axis of the magnet, the pole of which is 
used. In like manner, a magnetic pole has a tendency 
to revolve round such a wire. 

Illustration. Suppose the wire perpendicular, its upper end pos- 
itive, or attached to the positive pole of the voltaic battery, and its 
lower end negative ; and let the centre of a watch-dial represent 
the magnetic pole: if it be a north pole, the wire will rotate round 
it, in the direction that the hands move ; if it be a south pole, the 
motion will be in the opposite direction. From these two, the mo- 
tions which would take place if the wire were inverted, or the pole 
changed, or made to move, may be readily ascertained, since the 
relation now pointed out remains constant. 

Pig. 131 represents the ingenious apparatus, invented by Mr. 
Faradav, to illustrate the electro-magnetic rotation. The central 
pillar supports a piece of thick copper wire, which, on the one side, 

Fi „ 131 dips ' mXo the mer " 

cuxy contained in 

a small glass cup 
a. To a pin at the 
bottom of this cup, 
a small cylindri- 
cal magnet is at- 
tached by a piece 
of thread, so that 
one pole shall rise 
a little above the 
surface of the mer- 
cury, and be at 
liberty to move 
around the wire. 
The bottom of the 
cup is perforated, 
and has a copper 
pin passing through it ; w^hich, touching the mercury on the inside, 
is also in contact with the wire that proceeds outwards, on that side 
of the instrument On the other side of the instrument, b, the thick 
copper wire, soon after turning down terminates, but a thinner 
piece of wire forms a communication between it and the mercury 
on the cup beneath. As freedom of motion is regarded in the 
wire, it is made to communicate with the former by a ball and 
socket joint •, the ball being held in the socket by a piece of thread ; 
or else, the ends are bent into hooks, and the one is then hooked to 
the other. As good metallic contact is required, the parts should 
be amalgamated, and a small drop of mercury placed between them ; 
the lower ends of the wire should also be amalgamated. Beneath the 

What illustration is given ? What does fig. 131 represent ? Explain the figure ? 
How is the freedom of motion, which is required on the wire, obtained ! How 
can the metallic contact, which is required, be obtained ? 

17 




186 NATURAL PHILOSOPHY. 

hanging wire, a small circular magnet is fixed in the socket of the 
cup b, so that one of its poles is a little above the mercury. As in 
the former cup, a metallic connexion is made, through the bottom, 
from the mercury to the external wire. 

If now the poles of a battery be connected with the horizontal 
external wires, c, c, the current of electricity will be through the 
mercury and the horizontal wire, on the pillar which connects 
them, and it will now be found, that the moveable part of the wire 
will rotate around the magnetic pole in the cup b, and the magnetic 
pole round the fixed wire in the other cup a, in the direction before 
mentioned. 

By using a very delicate apparatus, the magnetic pole of the earth 
may be made to put the wire in motion. 

Fig. 132 represents another ingenious contrivance, invented by 
M. Ampere, for illustrating the electro-magnetic rotation ; and it 
has the advantage of comprising within itself the voltaic combina- 
Fig. 132. tion which is employed. It consists of a cylinder 
of copper, about two inches high, and a little less 
than two inches internal diameter, within which, 
is a smaller cylinder, about one inch in diameter. 
The two cylinders are fixed together by a bottom, 
having a hole in its centre, the size of the smaller 
cylinder, leaving a circular cell, which may be 
fiiled with acid. A piece of strong copper wire is 
fastened across the top of the inner cylinder, and 
from the middle of it, rises, at a right angle, a piece 
of copper wire, supporting a very small metal cup, 
containing a few globules of mercury. A cylin- 
der of zinc, open at each end, and about an inch 
and a quarter in diameter, completes the voltaic 
combination. To the latter cylinder, a wire bent 
like an inverted U, is soldered, at opposite sides ; 
and in the bend of this wire a metallic point is fixed, which, when 
inserted in the little cup of mercury, suspends the zinc cylinder in 
the cell, and allows it a free circular motion. An additional point 
is directed downwards from the central part of the stronger wire, 
which point is adapted to a small hole at the top of a powerful bar 
magnet. When the apparatus with one point only is charged with 
diluted acid, and set on the mag-net placed vertically, the zinc cvl- 
inder revolves in a direction determined by the magnetic pole which 
is uppermost. With two points, the copper revolves in one direc- 
tion, and the zinc in a contrary direction. 

If instead of a bar magnet, a hors-eshoe magnet be employ- 
ed, with an apparatus on each pole, similar to that which has now 
been described, the cylinders in each will revolve in opposite direc- 
tions. The small cups of mercury mentioned in the preceding 
description are sometimes omitted, and the points are inserted in an 
indentation on the inverted U. 

If the poles of a battery be connected with the horizontal external wires, c c, 
throughout, what will the current of electricity be.' Round what pole will tho 
movable part of the wire rotate? Hound what will the magnetic pole rotate? 
What does fig. 132 represent » Of what does it consist .' How will the cylinders 
in each revolve, if instead of a bar mainet a horse-sh f x» magnet be employed, with 
an apparatus on each pole similar to that which has now been described? 




ELECTRO-MAGNETISM. 187 

441. Magnets of prodigious power have been form- 
ed by means of Voltaic electricity. 

An electro-magnet was constructed by Professor Henry and Dr. 
Ten Eyck, which was capable of supporting a weight of 750 pounds. 
They have subsequently constructed another, which will sustain 
2063 pounds, or nearly a ton. It consists of a bar of soft iron, bent 
into the form of a horse shoe, and wound with twenty-six strands of 
copper bell-wire, covered with cotton threads, each thirty-one feet 
long ; about eighteen inches of the ends are left projecting, so that 
only twenty-eight feet of each actually surround the iron. The ag- 
gregate length of the coils is therefore 728 feet. Each strand is 
wound on a little less than an inch : in the middle of the horse shoe 
it forms, three thicknesses of wire; and on the ends, or near the 
poles, it is wound so as to form six thicknesses. Being connected 
with a battery consisting of plates, containing a little less than 48 
square feet of surface, the magnet supported the prodigious weight 
stated above, namely 2063 pounds. The effects of a larger battery 
were not tried. 

442. It is seen, by what has just been stated, that 
magnetism, of great power, is induced by electricity. 
It remains now to be stated that electricity, of consid- 
erable power, can be elicited from a magnet. 

This discovery was made, (as has already been stated) by Mr. 
Faraday. The experiment may be made in the following manner. 
Twelve sheer steel plates are to be connected in the form of a horse 
shoe; with a keeper or lifter made of the purest soft iron. Around 
the middle of the keeper is a wooden winder, having about a hun- 
dred yards of common thread bonnet wire, the two ends composed of 
four lengths of the wire twisted together, being carved out with a 
vertical curve of about three fourths of a circle; one of these twisted 
ends passing beyond each end of the keeper, and resting on the re- 
spective poles of the magnet. A small wooden lever is so fixed as 
to admit of the winder and the keeper being suddenly separated 
from contact with the magnet, when a beautiful and brilliant spark 
is perceived to issue from the extremity of the wire which first 
becomes separated from the magnet. By means of this electro- 
magnetic spark, gunpowder may be inflamed.* 

* A magneto-electrical machine has been constructed by Mr. J. Saxton, an in- 
genious mechanic of Philadelphia, resident in London. A similar apparatus has 
been made by Mr. J. Lukens, of Philadelphia, in a very neat and portable form, 
and it serves 10 demonstrate the nature of the reaction between magnets and elec- 
trical currents. Messrs. A. & D. Davis, of this city, have lately constructed an 
apparatus of the same kind, for Dr. Webster, Professor of Chemistry in Harvard 
University. 

Dr. Ritchie, Professor of Natural Philosophy in the University of London, has 

441. How have magnets of great power been formed? What weight was the 
magnet constructed by Professor Henry and Dr. Ten Eyck, capable of supporting? 
What weight will the one afterwards constructed sustain ? Of what dors it con- 
sist ? 442. By what is magnetism of great power induced? From what can 
electricity of considerable power be elicited » By whom was this discovery made! 
How can the experiment be made ? Who has succeeded in an attempt to cause 
the continued rotation of a temporary magnet ? How is this effect produced ? 



188 NATURAL PHILOSOPHY. 

The science of Electro-magnetism is yet in its infancy. The 
discoveries which have rewarded the laborers of philosophical 
research are truly wonderful ; — but man has as yet but lifted the 
veil, behind which the stupendous operations of nature are carried 
on. What wonders he will discover, should he penetrate the re- 
cesses of her laboratory, imagination cannot conceive. It would 
have excited no little surprise, among the philosophers of the last 
century, had the opinion been advanced, that Electricity and Mag- 
netism are identical. Perhaps the future philosopher may surprise 
a generation not very distant, by the annunciation of the discover}', 
that attraction and repulsion of all kinds, are to be traced to a 
common source, — that the same influence which binds the particles 
of a grain of sand together, is seen in the vivid flash which causes 
the " lit lake to shine ; " or heard in the " live thunder," as it leaps 
from peak to peak ; or known in the unerring guide which it fur- 
nishes the mariner in his course over the trackless deep ; and ad- 
mired in the music of the spheres, as they harmoniously roll in 
grand and magnetic course in the immeasurable regions of infinite 
space. 



SECTION XX. 

Astronomy. 

443. Astronomy treats of the heavenly bodies, such 
as the sun, moon, stars, comets, planets, &c. 

444. The earth on which we live is a large globe, or 
ball, nearly eight thousand miles in diameter, and about 
twenty-five thousand miles in circumference. It is 

succeeded in an attempt to cause the continued rotation of a temporary magnet 
on its centre, by the action of permanent magnets. This effect is produced by 
suddenly changing the poles of the temporary magnet, and thus, at the proper mo- 
ment, converting attraction into repulsion. 

Professor Henry, of Princeton, New Jersey, has constructed an apparatus for 
exhibiting, in a temporary magnet, a reciprocating motion. The soft iron mag- 
net, with its coils of wire, is suspended like the beam of a steam engine, on an ax- 
is, and furnished with projecting wires, which dip into mercurial cupp, connected 
with a voltaic battery at each end of the apparatus. The wires are so arranged 
as to change the poles of the soft magnet at every alternation in the movement. 
Each end of the soft iron bar plays between the poles of a permanent magnet, 
curved into an elliptical form ; and as it dips into the mercurial cup below, its po- 
larity is changed, and it is repelled. A vigorous action U thus kept up, which is 
limited only, by the durability of the materials in the galvanic circuit, and their 
power of furnishing a supply of electricity. 

43. Of what does astronomy treat? 444. What is said of the earth ? 



ASTRONOMY. 189 

known to be round — -Jirst, because it casts a circular 
shadow, which is seen on the moon, during an eclipse ; 
secondly, because the upper parts of distant objects on 
its surface, can be seen at the greatest distance ; thirdly, 
it has been circumnavigated. It is situated in the midst 
of the heavenly bodies, which we see around us at night, 
and forms one of the number of those bodies; and it 
belongs to that system, which, having the sun for its 
centre, and being influenced by its attraction, is called 
the solar* system. 

445. The solar system consists of the sun, which is 
in the centre. 

Of seven 'primary planets, named Mercury, Venus, the Earth, 
Mars, Jupiter, Saturn, and Herschel;! 

Of four Asteroids, or smaller planets, namely, Ceres, Pallas, Juno, 
and Vesta; 

Of eighteen secondary planets or moons, of which the Earth has 
one, Jupiter four, Saturn seven, and Herschel six; and 

Of an unknown number of comets. 

446. The stars, which we see in the night time, are 
supposed to be suns, surrounded by systems of planets, 
&c. too distant to be seen from the earth. Although 
they appear so numerous on a bright night, yet 
there are never more than a thousand visible to the 
sight, unassisted by glasses. The various refractions 
and reflections of the atmosphere make them appear 
much more numerous than they really are. 

447. Among the stars that are visible on a clear night 
may be seen a number which are called planets,^ [men- 
tioned } in No. 445.) The planets may be distinguished 
from the stars by their steady light ; while the stars are 
constantly twinkling. The planets, likewise, appear to 

* The word solar means belonging to the sun. 

■(• This planet is sometimes called Uranus,, and sometimes the Georgium Sidus. 

X The meaning of the word planet is properly a toanderer, or a wandering star. 
These luminaries were so called because they never retain the same situation, but 
are constantly changing their relative positions. While those slars which appear 
to retain their places are called fixed stars. The cause of the motion of the plan- 
ets will be presently explained. 

How is the earth known to be round.' Flow is it situated ? 445. Of what does 
the solar system consist » 446. What are the stars supposed to be ! How many 
are visible, on a bright night, unassisted by glas«es ? Why do they appear so nu- 
merous? 447. How may the planets be distinguished from the stars? How are 
the planets distinguished from xhe fixed stars ? What is the meaning of the word 
planet ' Why are they called planets? What are the fixed sts«-= > 

17* 



190 NATURAL PHILOSOPHY. 

change their relative places in the heavens, while those 
luminous hodies which are called fixed stars appear to 
preserve the same relative position. 

448. The sun, the moon, the planets, and the fixed 
stars, which appear to us so small, are supposed to be 
large worlds, of various sizes, and at different but im- 
mense distances from us. The reason that they appear 
to us so small is, that on account of their immense dis- 
tances they are seen under a small angle of vision. — 
[See Optics, page 125, No. 340.] 

449. It has been stated in the first part of this book, 
(See pages 13 and 14, Nos. 43, 44 and 45,) that every 
•portion of matter \s attracted by every other portion — 
and that the force of the attraction depends upon the 
quantity and the distance. On account of the immense 
distance of these heavenly bodies from the earth they 
will not fall, like other bodies, to its surface — and be- 
sides, if they were sufficiently near, the earth would 
rather fall upon one of them, because some of them are 
larger than the earth. As attraction, however, is mu- 
tual we find that all of the heavenly bodies attract the 
earth ; and the earth, likewise, attracts all of the heav- 
enly bodies. It has been proved, (see page 34, Nos. 126 
and J 27,) that a body when actuated by several forces 
will not obey either one, but will move in a direction be- 
tween them. It is so with the heavenly bodies — each 
one of them is attracted by every other one; and these 
attractions are so nicely balanced by creative wisdom, 
that, instead of rushing together in one mass, they are 
caused to move in regular paths, (called orbits,) around 
a central body; which being attracted in different direc- 
tions, by the bodies which revolve around it, will itself 
revolve around the centre of gravity of the system. 
Thus, the sun is the centre of what is called the solar 
system, (see No. 445,) and the planets revolve around 
it in different times, at different distances, and with dif- 
ferent velocities. [See No. 78, page 21.] 

448 H hat are the sun, moon, planets and fixed stars supposed to be > Why do 
they appear so small > 449. What has been staled with regard to the attraction 
of portions of matter ? Upon what does the force of this attraction depend! 
Why do not th<« heavenly bodies fall, like other bodies, to the surface of the earth ! 
What follows from attraction being mutual; What direction do bodies take when 
actuated by several forces ; Is this true with regard to the heavenly bodies ? What 
is meant by the orbit of a planet.' What is the centre of the solar system? 
What is said of the revolution of the planets? 



ASTRONOMY. 191 

450. The paths or courses in which the planets move 
around the sun are called their orbits. In obedience, 
therefore, to the universal law of gravitation, or gravi- 
ty, the planets revolve around the sun as the centre of 
their system; and the time that each one takes to per- 
form an entire revolution is called its year. Thus, the 
planet Mercury revolves around the sun in 87 of our 
days. Hence, a year on that planet is equal to 87 days. 
The planet Venus revolves around the sun in 224 days. 
That is, therefore, the length of the year of that planet. 
Our earth revolves around the sun in about 365 days 
and 6 hours. Our year, therefore, is of the same 
length. 

451. The length of time that each planet takes in 
performing its revolution around the sun, or, in other 
words, the length of the year on each planet is as fol- 
lows. {The fractional parts of the day are omitted.) 

Mercury 87 days. Vesta 1,325 days. Jupiter 4,332 days. 

Venus 224 " Juno .1,592 " Saturn 10,759 " 

Earth 365 " Ceres 1,681 " Herschel 30,686 " 

Mars 686 " Pallas 1,686 " 

452. The mean distance* of each of the planets from 
the sun is expressed as follows, in millions of miles. 

Millions. Millions. Millions. 

Mercury 36 " Vesta 225 " Jupiter 495 " 

Venus 68 " Juno 254 " Saturn 908 " 

The Earth 95 " Ceres 263 " Herschel 1,827 " 

Mars 145 " Pallas 264 " 

*The paths or orbits of the planets are not exactly circular, but elliptical. They 
are, therefore, sometimes nearer) o the sun than at others. The mean distance is 
the medium, between their greatest and least distance. Those planets which are 
nearer to the sun than ihe earth is, are oiled interior planets, because their orbits 
are within that of the earth— and those which are farther from the sun are called 
exterior planet3, because their oihits are outside that of the earth. Instead of in- 
terior and exterior, the names inferior and superior are sometimes used. 



450. What are the paths, in which the planets move around the sun, called ? 
Around what do the planets revolve ? What is a year on each planet I How long 
is the year of the planet Mercury .' flow long is the planet Venus performing her 
revolution around the sun ? How long is the earth in performing her revolution 
around the sun ? 451. What is the length of a year on the planet Mercury? 
Venus! Earth? Mars! Vesta! Juno? Ceres? Pallas? Jupiter? Saturn? Her- 
schel? 452. What is the distance of the planet Mercury from the Sun ! Venus! 
Earth? Mars! Ve3ta ' Juno! Ceres? Pallas? Jupiter? Saturn? Herschelj" Of 
what form are the orbits of the planets ? What is meant by the mean distance ? 
What planets are called interior? Why? What planets are called exterior? 
Why ? What other names are sometimes used ? 



192 



NATURAL PHILOSOPHY. 



453. While the planets revolve around the sun, each 
also turns around upon its own axis, and thus presents 
each side successively to the sun. The time in which 
they turn upon their axes is called their day, and is 
thus expressed in hours and minutes: 

H. M. H. M. 

Mercurv '24 • 5 " Vesta (unknown.) Jupiter 9 L 55 m. 
Venus " 20 2 23 (probably) Saturn 10 " 16 ' 

Eanh 22 Ceres (unknown. yHerschel (unknoicn.) 

24 " 39 " Pallas (unknoicn.) 
The sun turns on its axis in about 25 days and 10 hours. 

454. The relative size of the bodies belonging to the 
solar system, as expressed by the length of their diam- 
eters, is as follows : 

Miles. Miles. Itifca. 

The Sun 877.547 Mire 4.222 Pallas 

Mercurv 2.9*4 Vesta 269 Jupiter S»5.253 

Venus " 7.621 Juno 1.393 - -1.954 

Eanh 7.924 es 1.5-2 Herechel 34.363 

The : 1 I 180 miles. 
Kg 133 is a representation of the comparative size of the planets. 

The following illustration of the comparative size and distance of 
Fig- 133. 




"SS^ry liars *«!* -*«*J- 

o y o O (J 




Eerschel 



the bodies of the solar system is given by Sir J. F. W. Herschel. 
On a \rell levelled field place a globe, two feet in diameter, to rep- 
resent the Sun ; Mercury will be represented by a grain of mustard 



453. flare the planets any motion beside that around the sun : What is tbe 
time in vrhich tbey turn upon their axes called ? What is the length of a day oa 
the planet Mercury: VfnnaJ Earth. : Mars: Fatal Juno: Ceres? Pallas I Ju- 
piter: Saturn' He'rscnel . 454. What is the diameter of the Sun: Mercury i 

• ': Vesta ' Juno; Ceres: Pallas: Jupiter: Saturc 
scbel: The Moon : What does fig. 133 represent: What illustration of the 
comparative size and distance of the bodies of the solar system is given ? 



ASTRONOMY. 193 

seed on the circumference of a circle 164 feet in diameter for its 
orbit ; — Venus, a pea, on a circle 284 feet in diameter ; the Earth 
also a pea, on a circle of 430 feet ; Mars, a rather large pin's head, 
on a circle of 654 feet; Juno, Ceres, Vesta ana Pallas, grains of 
sand, in orbits of from 1000 to 1200 feet ; Jupiter, a moderate sized 
orange, in a circle nearly half a mile in diameter; — Saturn, a small 
orange, on a circle of four-fifths of a mile in diameter, and Herschel 
a full-sized cherry, or small plum, upon the circumference of a circle 
more than a mile and a half in diameter. 

"To imitate the motions of the planets in the above mentioned 
orbits, Mercury must describe its own diameter in 41 seconds ; 
Venus in 4 minutes and 14 seconds, the Earth in 7 minutes, Mars 
in 4 mirrutes and 48 seconds, Jupiter in 2 hours 56 minutes, Saturn 
in 3 hours 13 minutes, and Herschel 12 hours 16 minutes." 

455. The orbit of the earth is called the ecliptic. In 
other words, the ecliptic is the apparent path of the 
sun, or the real path of the earth. It is called the 
ecliptic, because every eclipse, whether of the sun or 
the moon, must be upon it. The zodiac is abroad space 
or belt, 16 degrees broad, 8 degrees each side of the 
ecliptic. It is called y he zodiac, from a Greek word, 
which signifies an am,\\xl, because all the stars in the 
twelve parts into which the ancients divided it, were 
formed into one sign or constellation, and most of the 
twelve constellations were called after some animal. 
The names of these constellations or signs are some- 
times given in Latin and sometimes in English. They 
are as follows : 

Latin. English. Latin. English. 

1 Aries The Ram. 7 Libra The Balance. 

2 Taurus The Bull. 8 Scorpio The Scorpion. 

3 Gemini The Twins. 9 Sagittarius The Archer. 

4 Cancer The Crab. 10 Capricornus The Goat. 

5 Leo The Lion. 11 Aquarius The Water-bearer. 

6 Virgo The Virgin. 12 Pisces The Fishes. 

Each sign or constellation contains 30 degrees of 
the great celestial circle.* 

456. The orbits of the other planets are inclined to 
that of the earth ; or, in other words, they are not in 
the same plane. 

♦Owing to the precession of the equinoxes, the stars which were formerly in 
the constellation called Aries, are now in the one called Taurus, &.c, each hav- 
ing gone forward one sign. 

What is necessary in order to imitate the motions of the planets in the above 
mentioned orbits? 455. What is the orbit of the earth called? What is the 
ecliptic ? Why is it called the ecliptic ? What is the zodiac ? Why is it called 
the zodiac? What are the names of the twelve constellations > How many de- 
gree! does each sign contain ? 456. Are the orbits of the other planets in the 
name plane with that of the earth .» 



194 



NATURAL PHILOSOPHY. 



Figure 134 represents an oblique view of the plane of the ecliptic, 
the orbits of all the primary planets, and of the comet of lbbU. i n<" 
Fig. 134. 




part of each orbit which is above the plane is shown by a white 
line ; that which is below it, by a dark line. That par t of the orbit 

What does fig. 134 represent ? 



ASTRONOMY. 



195 



of each planet 

the white and 

of the planet. 

Fig. 135. 




where it crosses the ecliptic, or, in other words,where 
dark lines in the figure meet, are called the nodes 
[From the Latin nodus, a knot or tie.] 

Figure 135 represents a section of the plane of the 
ecliptic, showing the inclination of the orbits of the 
planets. As the zodiac extends only eight degrees on 
each side of the ecliptic, it appears from the figure 
that the orbits of some of the planets are wholly in 
the zodiac, while those of others rise above and de- 
scend below it. Thus, the orbits of Juno, Ceres, and 
Pallas rise above, &c, while those of all the other 
planets are confined to the zodiac. 

When a planet or heavenly body is in that part of 
its orbit which is near any particular constellation, 
it is said to be in that constellation. Thus in Fig. 
134, the comet of 1680 appears to approach the sun 
from the constellation Leo. 

457. The perihelion* and aphelion* of 
a heavenly body express its situation with 
regard to the sun. When a body is nearest 
to the sun, it is said to be in its perihelion. 
When farthest from the sun, it is said to be 
in its aphelion. [See note to No. 452.] The 
earth is three millions of miles nearer to 
the sun in its perihelion, than in its aphe- 
lion. 

458. The apogee and perigee of a heav- 
enly body express its situation with re- 
gard to the earth. When the body is near- 
est to the earth, it is said to be in its per- 
igee ; when it is farthest from the earth, 
it is said to be in its apogee. 

459. The aphelia of the planets, or parts 
of their orbits in which they are nearest to 



* The plural of Perihelion is Perihelia, and of Aphelion is 
Aphelia. When a planet is so nearly on a line with (he eartli 
and the sun as to pass between them it is said to be in its infe- 
rior conjunction ; when behind the sun, it is said to be in ita 
superior conjunction ; but when behind the earth it is said to be 
in opposition. 



What are the nodes of a planet f What does fig. 136 represent? When is a 
planet said to be in any particular constellation .' 457. What do the perihelion 
and aphelion of a heavenly body express J When is a body said to be in its peri- 
helion I When is it said to be in its aphelion ! How much nearer is the earth to 
the sun in its perihelion than its aphelion i When is a planet said to be in its in- 
ferior conjunction ? When is it said to be in its superior conjunction ? When ia 
it said to be in opposition ? 457. What do the apogee and perigee of a heavenly 
body express? When is a body said to be in its perigee I When is it said to be k< 
its apogee ! 



196 NATURAL PHILOSOPHY. 

the sun (See note to No. 452) are in the following signs 
of the zodiac : — Mercury in Sagittarius — Venus in 
Aquarius — the Earth in Capricornus^ Mars in Virgo 
— Vesta in Cancer — Juno in Scorpio — Ceres in Pisces 
— Pallas in Aquarius — Jupiter in Libra — Saturn in 
Capricornus — and the Georgium Sidus in Aries * 

460 . The axes of the planets in their revolution 
around the sun, are not perpendicular to their orbits, 
nor to the plane of the ecliptic, but are inclined in dif- 
ferent degrees. 

This is one of the most remarkable circumstances in the science 
of Astronomy, because it is the cause of the different seasons, spring, 
summer, autumn and winter ; and because it is also the cause of the 
difference in the length of the days and nights in the different parts 
of the world, and at the different seasons of the year. 

461. The motion of the heavenly bodies is not uni- 
form. Their velocity is different in different parts of 
their orbits. They move with the greatest velocity when 
they are in perihelion, or in that part of their orbit 
which is nearest to the sun ; and slowest when in aphe- 
lion. 

It has been proved by Kepler, that in moving round a point to- 
wards which it is attracted, a body passes over equal areas in eqiial 
times. This is called Kepler's law. 

*The signs of the Zodiac and the various bodies of the solar system, are often 
represented in Almanacks and Astronomical works, by signs or characters. In 
the following list the characters of the planets &c. are represented. 

The Sun. © The Earth. $ Ceres. 

d The Moon. <f Mars. $ Pallas. 

§ Mercury. § Vesta. If Jupiter. 

9 Venus. 6 Juno. h Saturn. 

1$ Herschel. 

The following characters represent the signs of the Zodiac, 
rp Aries. zz Cancer. ±± Libra. 1£> Capricornus. 

y Taurus. Q Leo. TT\ Scorpio. ts. Aquarius. 

n Gemini. lift Virgo. f Sagittarius. >£ Pisces. 

From an inspection of the figure, it appears that when the earth, as seen from the 
sun, is in any particular constellation, the sun, as viewed from the earth, will ap- 
pear in the opposite one. 



459. In what sign is the aphelion of the planet Mercury ? Venus? Earth.' Mars? 
Vestaf Juno? Ceres? Pallas? Jupiter? Saturn.' Georgium Sidus.' 460. What 
is said with regard to tbe axes of the planets in their revolution around the sun ? 
What does this inclination of their axes cause; 461. What is said with regard 
to the motion of the heavenly bodies ? When do they move with the greatest ve- 
locity ? When is their motion the slowest ? What is Kepler's law ? 



ASTRONOMY. 



197 



Fig. 136. 



Illustration. In fig. 136, let S represent the Sun, and E the 
Earth and the ellipse or oval be the earth's orbit or path around the 
Sun ' Bv lines drawn from the Sun at S to the outer edge of the 

figure, the orbit is divid- 
ed into twelve areas (or 
parts) of different shapes, 
but each containing the 
same quantity of space. 
Thus, the spaces E S A, 
A S B, D S F, &c. are all 
supposed to be equal. Now 
ifthe earth, in the space of 
one month will move in its 
orbit from E to A, it will 
in another month move 
from A to B, and in the 
third month from B to C, 
&c; and thus will describe 
(or rather more properly- 
speaking, pass by) equal 
areas in equal times. 

The reason why the 
earth (or any other heav- 
enly body) moves with a 
greater degree of velocity 
in its perihelion, than in 
its aphelion, may likewise 




be explained by the same figure. 
The Earth in its progress from 



Thus: 
FtoL 



being constantly actuated 



by the sun's attraction, must, (as is the case with a stone when falling 
to the earth,) (See No. 110,) move with an accelerated motion. 
At L, the sun's attraction becomes stronger, on account of the near- 
ness of the earth ; and consequently in its motion from L to E, the 
earth will move with greater rapidity. At E, which is the perihe- 
lion of the earth, it acquires its greatest velocity. Let lis now de- 
tain it at E, merely to consider the direction of the forces by which 
it is actuated. If the sun's attraction could be destroyed, the force 
which has carried it from L to E, would carry it off in the dotted 
line from E to G, which is a tangent to its orbit. But while the Earth 
has this tendency to move towards G, the sun's attraction is contin- 
ually operating, "with a tendency to carry it to S. Now when a 
body is actuated by two forces, (See No. 1*26,) it will move between 
them; but as the sun's attraction is constantly exerted at right an- 
gles to the motion of the earth, the direction of the earth's motion 
will not be in a straight line, the diagonal of one large parallelo- 
gram, but through the diagonal of a number of infinitely small par- 
allelograms ; which, being united, form the curve line E A. 

It is thus seen, that while the earth is moving from L to E, and 
from E to A, the attraction of the sun is stronger than in any other 
part of its orbit, and will cause the earth to move rapidly. But in 



Illustrate this by fig. 126. Explain, by fig. 136, the reason why the earth, or 
tny other heavenly body, moves with a greater degree of velocity in its perihelion 
than in its aphelion i 

18 



198 NATURAL PHILOSOPHY. 

its motion from A to B, from B to C, and from C to F, the attraction 
of the sun, operating in an opposite direction, will cause its motion 
from the sun to be retarded until, at F, the direction of its motion is 
reversed, and it begins again to approach the sun. Thus, it appears 
that in its passage from the Perihelion to the Aphelion, the motion 
of the earth, as well as that of all the heavenly bodies must be con- 
stantly retarded— while in moving from their Aphelion to Perihe- 
lion, it is constantly accelerated; and at their Perihelion, their 
velocity will be the greatest. The earth therefore, is about seven 
days longer in performing the aphelinn part of its orbit, than in trav- 
ersing the perihelion part ; and the revolution of all the other plan- 
ets being the result of the same cause, is affected in the same manner 
as that of the earth. 

462. The central forces (see No. 129) which produce 
the revolution of the heavenly bodies around a common 
centre, are both the result of gravitation. The attrac- 
tion of the sun is the centripetal force. The attraction 
of the other heavenly bodies, such as the planets, and 
even the very remote stars and comets, operating in a 
different direction, is the centrifugal force.* [See No. 
IS, page 21, and No. te§,page 190.] 

463. The earth is about three millions of miles near- 
er to the sun in winter than in summer. The heat of 
summer, therefore, cannot be caused by the near ap- 
proach of the earth to the sun. 

Snow and ice never melt on the tops of high mountains; and they 
who have ascended in the atmosphere, in balloons, have found that 
the cold increases as they rise. 

464. On account of the inclination of the earth's 
axis, (see No. 460) the rays of the sun fall more or less 
obliquely on different parts of the earth's surface, at 
different seasons of the year. The heat is always the 
greatest when the sun's rays fall vertically, that is, per- 
pendicular; and the more obliquely they fall, the less 
heat they appear to possess. 

* In many treatises on this subject, mention is made of a projectile force. As, 
however, all the harmonious motions, revolutions, &c , of the various bodies can 
be satisfactorily explained on the principle of gravitation alone, and as the use of 
the word projectile is obnoxious to th« objection that it conveys a misrepresenta- 
tion of the truth, it has in this work been purposely avoided. 

What is said of the motion of the heavenly bodies from perihelion to aphelion .' 
What is their motion from aphelion to perihelion: When is tin ir velocity the 
greatest! How much longer is the earth in performing the aphelion part of its 
orbit than the perihelion part .' 4G2. Of what are the central forces, which pro- 
duce the revolution of the heavenly bodies around a common centre, the result? 
What is the centripetal force? What is the centiifugal force? 463. How much 
nearer is the earth to the sun in winter than in summer? 464. What follows from 
the inclination of the earth's axis, with regard to the direction of the sun's rays ? 
When is the heat always the greatest .' What is said of oblique rays ? 



ASTRONOMY. 



199 



This is the reason why the days are hotter in summer, although 
the earth is farther from the sun at that time. 

Illustration. Fig. 137 represents the manner in which the rays 
of the sun fall upon the earth in summer and in winter. The north 
pole of the earth, at all seasons, constantly points to the north star, 
N ; and when the earth is nearest to the sun, the rays from the sun 
fall as indicated by W, in the figure ; and as their direction is very 
oblique, and they have a larger portion of the atmosphere to tra- 
verse, much of their power is lost. Hence we have cold, weather 

Fig. 137. 




when the earth is nearest to the sun. But, when the earth is in 
aphelion, the rays fall almost vertically, or perpendicularly ; and, 
although the earth is then nearly three million of miles further from 
the sun, the heat is greatest, because the rays fall more directly, and 
have a less portion of the atmosphere to traverse. * 

For a similar reason, we find, even in summer, that early in the 
morning, and late in the afternoon, it is much cooler than at noon ; 
because the sun then shines more obliquely. The heat is generally 
the greatest at about three o'clock in the afternoon ; because the earth 
retains its heat for some length of time, and the additional heat it is 
constantly receiving from the sun, causes an elevation of tempera- 
ture, even after the rays begin to fall more obliquely. 

*This may be more familiarly explained, by comparing summer rays to 'a ball 
or stone thrown directly at an object, so as to strike it with all its force; — and 
winter rays to tho same ball or stone, thrown obliquely, so as merely to graze the 
object. 



What is the reason that the heat is greater in 3ummerthan in winter? Illustrate 
this by fig. 137. How is the earth situated with regard to its distance from the 
sun in winter > What illustration of oblique and perpendicular rays is given in the 
note? Why is it generally cooler early in the morning and late in the afternoon 
than at noon >. Why is the heat the greatest at about three o'clock ? 



200 NATURAL PHILOSOPHY. 

It is ihe same cause which occasions the variety of climate 
in different parts of the earth. The sun always shines in a direc- 
tion nearly perpendicular or vertical on the equator; and with dif- 
ferent degrees of obliquity on the other parts of the earth. For this 
reason, the greatest degree of heat prevails at the equator during the 
whole year. The farther any place is situated from the equator, 
the more obliquely will the rays fall, at different seasons of the year; 
and consequently the greater will be the difference in the tempera- 
ture. 

465. If the axis of the earth were perpendicular to its 
orbit, those parts of the earth which lie under the equa- 
tor would be constantly opposife to the sun ; and 
as, in that case, the sun would at all times of the year 
be vertical to those places equally distant from hoth 
poles ; so the light and heat of the sun would be 
dispersed with perfect uniformity towards each pole ; 
we should have no variety of seasons; day and night 
would be of the same length ; and the heat of the sun 
would be of the same intensity every day throughout 
the year. 

It is, therefore, as has been stated, owing to the inclination of the 
earth's axis, that we have the agreeable variety of the seasons, days 
and nights of different lengths, and that wisely ordered variety of 
climate, which causes so great a variety of productions, and which 
has afforded so powerful a stimulus to human industry.* 

466. In order to understand the illustration of the causes 
of the seasons, &c. it is necessary to have some knowl- 
edge of the circles which are drawn on the artificial 
representations of the earth. It is to be remembered 
that all of these circles are wholly imaginary — that is, 
that there is on the earth itself no such circles or lines. 
They are drawn on maps merely for the purpose of il- 
lustration. 

*Tlie wisdom of Providence is frequently displayed in apparent inconsistencies. 
Thus, the very circumstances which to the' shortsighted philosopher appears to 
have thrown an insurmountable harrier betvven ihe scattered portions of the hu- 
man race, has been wisely ordered to establish an interchange of blessings, and to 
bring the ends of the earth in communion. Were the same productions found in 
every region of the earth, the stimulus to exertion would be weakened, and the 
wide field of human labor would be greatly diminished. It is our mutual wants 
which bind us together. 

What causes the variety of climate in different parts of the earth > Where does 
the sun always shine in a vertical direction? 465. What would follow were the 
axis of the earth perpendicular to its orbit? What causes the variety of the sea- 
sons, the different lengths of days and nights, &c. 466. What is necessary in 
order to understand the illustration of the.causes of the seasons ? 



ASTRONOMY. 



201 





Pig. ] 


38. 








%-- 


^x£v 




*/ 










\& 


c(— ■ 






















"*\ 










/4 




<r 


£ 




Y 





Pig. 139 represents the earth. N S is the axis, or imaginary- 
line, around which it daily turns; N is the north pole, S is the south 
pole. These poles, it will be seen, 
are the extremities of the axis, N S. 
C D represents the equator, which 
is a circle around the earth, at an 
equal distance from each pole. — 
The curved lines proceeding from 
N to S, are meridians. They are 
all circles surrounding the earth, 
and passing through the poles. — 
These meridians may be multipli- 
ed at pleasure. 

The lines E F, I K, L M, and 
G H, are designed to represent 
circles, all of them parallel to the 
equator, and for this reason they 
are called parallels of latitude. These also may be multiplied at 
pleasure. 

But in the figure, these lines, which are parallel to the equator, 
and which are at a certain distance from it, have a different name, 
derived from the manner in which the sun's rays fall on the sur- 
face of the earth. 

Thus the circle I K, 23£ degrees from the equator is called the 
tropic of Cancer, because, when the sun's vertical rays fall upon that 
portion of the earth, they turn* and proceed backwards towards 
the equator. 

For a similar reason, the circle L M is called the tropic of Cap- 
ricorn.! The circle E F is called the Arctic Circle. It represents 
the limit of perpetual day, when it is summer in the northern hem- 
isphere, and of perpetual night when it is winter. 

The circle G H is the Antarctic Circle, and represents the limit of 
perpetual day and night in the southern hemisphere. The line L K, 
represents the circle of the ecliptic, which, (as has already been sta- 
ted in No. 455,) is the apparent path of the sun, or the real path of 
the earth. This circle, although it is generally drawn on the terres- 
trial globe, is, in reality, a circle in the heavens ; and differs from 
the zodiac only in its width— the zodiac extending eight degrees on 
each side of the ecliptic. [See No. 455, page 193.] 

Fig. 139 represents the manner in which the sun shines on the 
earth in different parts of its orbit; or, in other words, the cause of 
the change in the seasons. S represents the sun, and the dotted 
oval, or ellipse, A B C D, the orbit of the earth. The outer circle 

* The word tropic is derived from a word which mean9 to turn. 

f The tropics, therefore, are the boundaries of the sun's apparent path, north 
and south of the equator. 



Explain fi». 138. What are the poles? Why is the circle I K called the tropic 
of Cancer* What is the meaning of the word tropic' Why is the circle L M 
called the tropic of Capricorn ? What are the tropics? What is the circle EP 
called ? What does it represent J What is the circle G H called ? What does it 
represent ? 

18* 



202 



NATURAL PHILOSOPHY. 



represents the zodiac, with the position of the twelve sisns or con- 
stellations, On the 21st of June, when the earth is at D, the whole 
northern polar region is continually in the light of the sun. As it 

Fiff. 139. 




W alDay **% 






vm \Summe r in ^K Summer m 

„J%27| (he Northern % S gikeSouthen 

^^Kemisphere ''"'-' Mem isphere 



'9**1 Day&l 





turns on its axis, therefore, it will be day to all the parts which are 
exposed to the light of the sun.* But, as the whole of the Antarctic 
Circle is within the line of perpetual darkness, the sun can shine on 
no part of it. It will, therefore, be constant night to all places witb- 

* Day and night are caused by the rotation of the earth on its axis every 24 
hours. It is day to that side of the earth which is towards the sun, and night to 
the opposite side. The length of the days is in proportion to the inclination of 
the axis of the earth loicards the sun. It may be seen by the above figure, that 
in summer, the axis is most inclined towards the sun, and then the days are the 
longest. As the axis becomes less inclined, the days shorten, til)} on the 21st De- 
cember, it is inclined 23 1-2° from the sun/ when the days are the shortest. Thu?> 
as the earth progresses in its oibit, after the days are the shortest, it changes its 
inclination toward the Sun, till it is again inclined as in the longest days in the 
summer. 

* The engraver has misrepresented the line of perpetual darkness on the earth, 
at B and D. It should extend from the tropic of Cancer to the tropic of Capri- 
corn ; whereas, in the figure, it appears to extend from the North to the South 
pole. The mistake was not discovered until it was too late to correct it in this 
edition. 

What does fig. 139 represent? Explain the figure. Explain, by the figure, the 
situation of the earth on the 21st of June. What causes day and night? To 
what part of the earth is it day? To what part is it night; To what is the length 
of the day in proportion ? VVhen are the days the longest » Why > When are 
they the shortest J Why i 



ASTRONOMY. 203 

in that circle. As the whole of the Arctic Circle is within the 
line of perpetual light, no part of that circle will be turned from 
the sun while the earth turns on its axis. To all places, therefore, 
within the Arctic Circle, it will be constant day. 

On the 22d of September, when the earth is at C, its axis is nei- 
ther inclined to, nor from the sun, but is sideways ; and, of course, 
while one half of the earth, from pole to pole, is enlightened, the 
other half is in darkness, as would be the case if its axis were per- 
pendicular to the plane of its orbit; and it is this which causes the 
days and nights, of this season of the year, to be of equal length. 

On the 23d of December, the earth has progressed in its orbit to 
B, which causes the whole space within the northern polar circle to 
be continually in darkness, and more of that part of the earth north 
of the equator to be in the shade than in the light of the sun. 
Hence, on the 21st of December, at all places north of the equator, 
the days are shorter than the nights, and at all places south of the 
equator, the days are longer than the nights. Hence, also, within 
the Arctic Circle it is uninterrupted night, the sun not shining at all ; 
and within the Antarctic Circle it is uninterrupted day, the sun 
shining all the time. 

On the 20th of March, the earth has advanced still further, and is 
at A, which causes its axis, and the length of the days and nights 
to be the same as on the 20th of September.* 

From the explanation of figure 139, it appears that there are two 
parts of its orbit in which the days and nights are equal all over 
the earth. These points are in the sign of Aries and Libra, which 
are therefore called the equinoxes. Aries is the vernal (or spring) 
equinox, and Libra the autumnal equinox. 

There are also two other points called solstices, because the sun 
appears to stand at the same height in the heavens, in the middle of 
the day, for several days. These points are in the signs Cancer 
and Capricorn. Cancer is called the summer solstice, and Capri- 
corn the winter solstice. 

* As the difference in the length of the days and the nights, and the change of 
the seasons, &c. on the earth, is caused by the inclination of the earth's axis, it 
follows that all the planets, whose axes are inclined, must experience the same 
vicissitude ; and that it must be in proportion to the degree of the inclination of 
their axes. As the axis of the planet Jupiter is nearly perpendicular to its orbit, 
it follows that there can be little variation in the length of the days, and little 
change in the seasons of that planet. 

There can be little doubt that the sun, the planets, stars. &.c. are all of them 
inhabited ; and although it may be thought that some of them, on account of 
their immense distance from the sun, experience a great want of light and heat, 
while nthers are so near, and the heat, consequently, so groat that water cannot 
remain on them in a fluid state, yet, as we see, even on our own earth, that creat- 
ures of different nature live in different elements, as, for instance, fishes in water, 
animals in air, &.c. creative wisdom could, undoubtedly, adapt the being to its sit- 
uation, and with as little exertion of power, form a race whose nature should be 
adapted to the nearest, or the most remote of the heavenly bodies, as was required 
to adapt the fowls to the air, or the fishes to the sea. 

Explain Tiy the figure the situation of the earth on the 22d of September.. On 
the 23d of December. On the 20th of March. What fellows from the changes 
on the earth, caused by the inclination of the earth's axis? In what proportion 
are these changes .' What is said of the axis of the planet Jupiter? Is it sup- 
posed that the sun, planets and stars are inhabited ! What is shown by fig. 139 ? 
Where are these points > What are they called ? Which is the vernal equinox? 
Which the autumnal ? What other two points are there? Whv are they called 
solstices ? Where are these point* .' Which is the summer solstice ? Which the 
winter I 



204 NATURAL PHILOSOPHY. 

467. The sun is a spherical body, situated near the 
centre of gravity, of the system of planets of which our 
earth is one. Its diameter is 877,547 English miles; 
which is equal to 100 diameters of the earth ; and, there- 
fore, his cubic magnitude must exceed that of the earth 
one million of times. It revolves around its axis in 25 
days and 10 hours. This has been ascertained by means 
of several dark spots which have been seen with tele- 
scopes on its surface. 

Dr. Herschel supposed the greater number of spots on the sun to 
be mountains; some of which he estimated to be 300 miles in height. 

It is probable that the sun, like all the other heavenly bodies (ex- 
cepting perhaps comets) is inhabited by beings whose nature is 
adapted to their peculiar circumstances. 

Although, by some, the sun is supposed to be an immense ball of 
fire, on account of the effects produced at the distance of ninety-five 
millions of miles, yet many facts show that heat is produced by the 
sun's rays, only when they acton a suitable medium. Thus, snow 
and ice remain during the year, on the tops of the highest moun- 
tains, even in climates where the cold of our winters is never known. 

It is supposed, by some astronomers, that the sun and planets have 
a general motion with relation to the fixed stars; and that their mo- 
tion is at the rate of the earth's motion in its orbit. But at this rate, 
if the distance of the stars is two hundred thousand times that of the 
diameter of the earth's orbit, they would be sixty thousand years in 
moving over the distance of the nearest fixed star. 

The zodiacal light is a singular phenomenon, accompanying the 
sun. It is a faint light which often appears to stream up from the 
sun a little after sunset and before sunrise. It appears nearly in the 
form of a cone, its sides being somewhat curved, and generally but 
ill defined. It extends often from 50° to 100° in the heavens, and 
always nearly in the direction of the plane of the ecliptic. It is 
most distinct about the beginning of March; but is constantly visi- 
ble in the torrid zone. The cause of this phenomenon is not known. 
In almanacs, the sun is usually represented by a small circle, with 
the face of a man in it, thus : (v) 

468. Mercury is the nearest planet to the sun, and 
is seldom seen ; because his vicinity to the sun occasions 
his being lost in the brilliancy of the sun's rays. 

The heat of this planet is so great that water cannot exist there, 
except in a state of vapor ; and metals would be melted. The in- 

467. What is said of the sun ? What is its diameter ? How much does its c»- 
bic magnitude exceed that of 1 he earth .' How long is it in performing its revolu- 
tion around its axis.' How has this been ascertained > What did Dr. Herschel 
suppose these spots to be? What is supposed by some astronomers of a general 
motion of ihe sun and planet? with relation to the fixed stars.' What is The Zo- 
diacal light .' At what time is t most distinct? Where is it constancy visible? 
468. What planet is nearest to the sun.' Why is it seldom seen? What isjsaid of 
the heat of ibis planet ? 



ASTRONOMY 205 

tenseness of the sun's heat, which is in the -ame proportion as its 
light, is seven times greater in Mercury than n the earth • so that 
water there would be carried off in the shape >f steam ; for, by ex- 
periments made with a thermometer, it appea- s that a heat seven 
times greater than that of the sun's beams in summer, will make 
water boil. 

Mercury, although in appearance only a small j a r, emits a bright 
white light, by which it maybe recognized when-een. It appears 
a little before the sun rises, and again a little after si-iset, but is nev- 
er to be seen longer than one hour and fifty minut s after sunset ; 
nor longer than that time before the sunrises. 

When viewed through a good telescope, Mercury appears with 
all the various phases, or increase and decrease of ligh w ith which 
we view the moon ; except that it never appears quite i\\ } because 
its enlightened side is turned directly towards the t r th, only 
when the planet is so near the sun as to be lost to our Srht in its 
beams. Like that of the moon, the crescent or enlightene sic i e f 
Mercury is always toward the sun. As no spots are comm^iy vis- 
ible on the disk, the time of its rotation on its axis is unkno u . 

469. Venus,* the second planet in order from the^n, 
is the nearest to the earth, and on that account app^ rs 
to be the largest and most beautiful of all the plants. 
During a part of the year it rises before the sun, andt 
is then called the morning star; during another parte: 
the year it rises after the sun and it is then called the 
evening star. The heat and light at Venus are nearly 
double what they are at the earth. 

As the orbits of Mercury and Venus are both within that of the 
earth, neither of those planets can ever appear at a greater distance 
than 90 degress, or a quarter of a circle from the sun. 

These two planets sometimes pass directly between the sun and 
the earth. As their illuminated surface is toward the sun, their 
dark side is presented to the earth, and they appear like dark spots 
on the sun's disk. This is called the transit of those planets. 

Venus, like Mercury, presents to us all the appearances of in- 
crease and decrease of light common to the moon. Spots are also 
sometimes seen on its surface, like those on the sun. By reason of 

* By the ancient Poet?, Venus was called Phosphor, or Lucifer, when it appear- 
ed to the west of ihe sun. at which time it is morning star, and ushers in the 
light or day; and Hesperus or Vesper, when eastward of the sun, or evening star. 

How much greater is the sun's heat in Mercury than on the earth ? In what 
form does water exist in Mercury ? How can Mercury be recognized when seen? 
At what time does it appear; How does Mercury appear when viewed through a 
telescope! 469. What planet is nearest to the earth? When is Venus called the 
morning star! Wl ten is it called the evening star "? How much greater are tho 
heat and light at Villus than that at the earth? What name was given by the 
ancient poets, to Venus, when morning star ? What, when evening star ? What 
is the greatest distance at which the planets, Mercury and Venus, can ever appear 
from the sun? Why! What is meant by the transit of these planets! What 13 
aaid of the different appearances which Venus presents! 



206 natufal philosophy. 

the great brilliancy of ths planet it may sometimes be seen even in 
the day time, by the na-'ed eye.* In the absence of the moon it will 
cast a shadow behind V opake body. 

470. The eartl is the next planet, in the solar sys- 
tem, to Venus. Jt is not a perfect sphere, but its fig- 
ure is that of ai oblate spheroid, the equatorial diame- 
ter being abott 34 miles longer than its polar diameter. 
It is attendee^/ one moon, the diameter of which is 
about two tiousand miles. Its mean distance from 
the earth isabout 240 thousand miles, and it turns on 
its axis in^recisely the same time that it performs its 
revolution round the earth; namely, in 29 days and a 
half. H^ce, it appears that the moon always presents 
the sam* side to the earth. The earth, when viewed 
from t^ moon, exhibits precisely the same phases that 
the rry° n does to us, but in opposite order. When the 
moo-is full to us, the earth will be dark to the inhab- 
itaps of the moon ; and when the moon is dark to us, 
thfearth will be full to them. The earth appears to 
thm about 13 times larger than the moon does to us. 
jfi the moon, however, always presents the same side 
j the earth, there is one half of the moon which we 
never see, and which cannot see the earth. 

471. Next to the earth is the planet Mars. It is con- 
spicuous, for its fiery red appearance ; which is suppos- 
ed to be caused by a very dense atmosphere, visible 
through a telescope; so that when this planet approach- 
es any of the fixed stars, they change their color, grow 
dim, and often become totally invisible. The degree of 
heat and light at Mars is less than half of that received 
by the earth. 

472. The four small planets or asteroids, Vesta, Ju- 
no, Ceres and Pallas, have all been discovered within 

* The reason why we cannot see the stars and planets in the day time, is, that 
their light is so faint, compared wiih the light of the sun reflected by our at- 
mosphere. 

Why can we not see iho planets and stars in the day time ? 470. What planet 
is next to Venus ? What is ihe form of the earth? How much larger is its equa- 
torial diameter than its polar ? How many moons has the earth ? What is the 
diameter of the moon ? What is its distance from the earth? What is the length 
of a day at the moon ? How long is it in performing its revolution around the 
earth >. What phases does the earth, when viewed fmrn the moon, exhibit? How 
much larger does the earth appear than the moon ? 471. What planet is next to 
the earth ? What renders it conspicuous ? What is supposed to cause this appear- 
ance? How much more light and heat does the earth enjoy than Mars? 479. 
When were the asteroids discovered t 



ASTRONOMY. 207 

the present century. Vesta was discovered by Dr. 01- 
bers, of Bremen, in 1807. Its light is pure and white. 
Juno, by Mr. Ffarding, near Bremen, in 1804. Its col- 
or is red, ami its atmosphere appears cloudy. Pallas 
was discovered by Dr. Olbers in 1802. It appears to 
have a dens^ cloudy atmosphere. Ceres was discover- 
ed at Palermo, in Sicily, by Piazzi, in 1801. It is of a 
ruddy coL>r. All of these small planets undergo vari- 
ous changes in appearance and size, so that their real 
magnitide is not ascertained with any certainty ; and 
but lit'le is known of them.* 

47S. Jupiter is the largest planet of the solar system, 
and it is the most brilliant, except Venus. The heat 
and light at Jupiter is about 25 times less than that at 
the earth. This planet is attended by four moons, or 
satellites; the shadows of some of which are occasion- 
ally visible upon his surface. 

The distance of those satellites from the planet are two, four, six 
and twelve hundred thousand miles, nearly. 

The nearest revolves around the planet in less than two days ; 
the next in less than four days ; the third in less than eight days ; and 
the fourth in about sixteen days. 

These four moons must afford considerable light to the inhabitants 
of the planet ; for the nearest appears to them four times the size of 
our moon; the second about the same size; the third somewhat 
less, and the fourth about one third the diameter of our moon. 

As the axis of Jupiter is nearly perpendicular to its orbit, it has 
no sensible change of seasons. 

The satellites of Jupiter often pass behind the body of the plan- 

* It is a remarkable fact, that certain irregularities, observed in the motions of the 
old planets, induced some astronomers to suppose that a planet existed between 
the orbits of Mars and Jupiter; a supposition that arose long previous to (he dis- 
covery of the four new planets just noticed The opinion has been advanced, 
that these four small bodies originally comp3sed one larger one, which, by some 
unknown for^.e or convulsion, burst asunder. This opinion is maintained with 
much ingenuity and plausibility by Dr. Brewster, in the Edinburgh Encyclope- 
dia, Art. Astronomy. Dr. Brewster further supposes, that the bursting of this 
planet may have occasioned the phenomena of the meteoric stones; that is, stonee 
which have fallen on the earth from the atmosphere. 

By whom, and in what year was Vesta discovered? What is the color of its 
light.' By whom and when was Juno discovered ? What is the color of its light? 
When was Pallas discoveied >. By whom? What is said of its atmosphere ? When 
and by whom was Ceres discovered? What is its color? What is said in the 
note with regard to these planets? 473. Which of ihe p'anets is the largest.' 
How mu'h more light and heat does the earth enjoy than Jupiter ? How many 
moons has this planet ? What is the distance of these moons from the planet >. 
In what time do they perform their revolutions around the planet ? How does the 
size of these moons compare with that of ours ? Why has Jupiter no sen- 
Bible variety of seasons .' 



208 NATURAL PHILOSOPHY. 

et, and also into its shadow, and are eclipsed. These eclipses are of 
use in ascertaining the longitude of places on the earth. By these 
eclipses also, it has been ascertained that light ia about 8 minutes in 
coming from the sun to the earth. For, an eclipse of one of these 
satellites appears tons to take place 16 minutes sooner, when the 
earth is in that part of its orbit nearest Jupiter, ban when in the 
part farthest from the planet. Hence, light is sixeen minutes in 
crossing the earth's orbit, and, of course, half of 'hat time, or 8 
minutes, in coming from the sun to the earth. 

When viewed through a telescope, several belts or binds are dis- 
tinctly seen, sometimes extending across his disk, and sometimes in- 
terrupted and broken. They differ in distance, position, and num- 
ber. They are generally dark; but white ones have been seen. 

On account of the immense distance of Jupiter from the sun, and 
also from Mercury, Venus, the Earth and Mars, observers en Jupi- 
ter, with eyes like ours, can never see either of the above earned 
planets, because they would always be immersed in the sun's lays. 

474. Saturn is the second in size and the last but one 
in distance from the sun. The degree of heat ard 
light at this planet is eighty times less than that at the 
earth. 

Saturn is distinguished from the other planets by being encom- 
passed by two large luminous rings, one exactly without or beyond 
the other. They reflect the sun's light in the same manner as his 
moons. They are entirely detached from each other and from the 
body of the planet. They turn on the same axis with the planet, 
and in nearly the same time* The edge of these rings is constant- 
ly at right angles with the axis of the planet. Stars are sometimes 
seen between the rings, and also between the inner ring and the 
body of the planet. The breadth of the two rings is about the same 
as their distance from the planet, namely, 21,000 miles. As they 
cast shadows on the planet, Dr. Herschel thinks them solid. 

The surface of Saturn is sometimes diversified, like that of Jupi- 
ter, with spots and belts. Saturn has seven satellites, or moons, re- 
volving around him at different distances and in various times, 
from less than one to eighty days. 

Saturn may be known by his pale and steady light. The seven 
moons of Saturn, all, except one, revolve at different distances 
around the outer edge of his rings. Dr. Herschel saw them mov- 

* These rings move together around the planet, but are about thirteen minutes 
longer in performing their revolution about him, than Saturn is in revolving about 
his axis. 

Of what use are the eclipses of Jupiter's moons.' How long is light in com- 
ing from the sun to the earth ? How has this been ascertained ? How does Ju- 
piter appear when viewed through a telescope i 474. How does Saturn compare 
in size with the othei planets ? How is Saturn distinguished from the other plan- 
ets? What is said of these rings i llow much longer are these rings in perform- 
ing their revolution around the planet than the planet is in performing its revolu- 
tion on its f>xis ? What is the breadth of these rings ? What is said of the sur- 
face of Saturn ? How many moons has Saturn I How may Saturn be known ? 
What is said of the moons of Saturn? 



ASTRONOMY. 209 

ing along it, like bright beads on a white string. They do not often 
suffer eclipse by passing into the shadow of the planet, because the 
ring is generally in an oblique direction. 

475. Herschel, the third in size, is the most remote 
of all the planets. It is scarcely visible to the naked 
eye. The light and heat at Herschel are about 360 
times less than that at the earth. 

This planet was formerly considered a small star, but Dr. Her- 
schel, in 1781, discovered from its motion that it is a planet. He 
modestly gave it the name of Georgium sidus, or the Georgium star, 
in honor of his King, George the Third. On the continent of Eu- 
rope it is called Uranus. 

Herschel is attended by six moons, or satellites; all of which were 
discovered by Dr. Herschel, and all of them revolve in orbits near- 
ly perpendicular to that of the planet. Their motion is apparently 
retrograde; but this is probably an optical illusion, arising from the 
difficulty of ascertaining which part of their orbit inclines towards 
the earth, and which declines from it.* 

It is a singular circumstance that, before the discovery of Her- 
schel, some disturbances and deviations were observed by astrono- 
mers in the motions of Jupiter and Saturn, which they could ac- 
count for only on the supposition that these iwo planets were influ- 
enced by the attraction of some more remote and undiscovered plan- 
et. The discovery of Herschel completely verified their opinions, 
and shows the extreme nicety with which astronomers observe the 
motions of the planets. 

476. The word comet is derived from a Greek word, 
which means hair; and this name is given to a numer- 

* It appears to be a general law of satellites, or moons, that they turn on their 
axes in the same time in which they revolve around their primaries. On this ac- 
count, the inhabitants of secondary planets observe some singular appearances, 
which the inhabitants of primary planets do not. Those who dwell on the side 
of a secnndary planet next to the primary will always see that primary ; while 
those who live on the opposite side will never see it. Those who always see the 
primary, will see it constantly in very nearly the same place. For example, those 
who dwell near the edge of the moon's disk will always see the earth near the ho- 
rizon, and those in or near the centre will always see it directly or nearly over 
head. Those who dwell in the moon's south limb will see the earth to the north- 
ward; those in the north limb will see it to the southward ; those in the east 
limb will see it to the westward ; while those in the west limb will see it to the 
eastward ; and all will see it nearerthe horizon inproportion to their own distance 
from the centre of the moon's disk. Similar appearances are exhibited to the in- 
habitants of all secondary planets. 



Why are they not often eclipsed .' 475. How does Herschel compare in size 
with the other planets ? How does the light and heat at Herschel compare with 
that of the earth.' By whom was this planet discovered > What name did he 
give it >. How many moons has Herschel? By whom were they discovered ? . How 
are their orbits situated with regard to that of the planet ? What is said of their 
motion ? What, appears to be a general law of satellites ? What follows from this 
with regard to the appearances which the inhabitants of the secondary planets 
must observe >. 476. What is the meaning of the word comet! 

19 



210 NATURAL PHILOSOPHY. 

ous class of bodies which occasionally visit, and appear 
to belong to the solar system. These bodies appear to 
consist of a nucleus, attended with a lucid haze, some- 
times resembling flowing hair ; from whence the name 
is derived. Some comets seem to consist wholly of 
this hazy or hairy appearance, which is frequently 
called the tail of the comet. 

In ancient times the appearance of comets was regarded with 
superstitious fear, in the belief that they were the forerunners of 
some direful calamity. These fears have now been banished, and 
the comet is viewed as a constituent member of the system, govern- 
ed by the same harmonious and unchanging laws which regulate 
and control all the other heavenly bodies * 

Comets in moving, describe long, narrow ovals. They approach 
very near the sun in one of the narrow ends of these ovals ; and 
when a comet is in the other, or opposite end of its orbit, its distance 
from the sun is incalculably great. 

The extreme nearness of approach to the sun, gives to the comet, 
when in perihelion, a swiftness of motion prodigiously great. 
Newton calculated the velocity of the comet of 1680, to be 880,000 
miles an hour. This comet was remarkable for its near approach 
to the sun ; being no further than 580.000 miles from it ; which is but 
little more than half the sun's diameter. Brydone calculated, that 
the velocity of a comet which he observed at Palermo in 1770, was 
at the rate of two millions and a half of miles in an hour. 

The luminous stream, or tail of a comet, follows it as it approach- 
es the sun, and goes before it when the comet recedes from the sun. 
Newton and some other astronomers considered the tails of com- 
ets to be vapors, produced by the excessive heat of the sun. Of 
whatever substance they may be, it is certain that it is very rare, be- 
cause the stars may be distinctly seen through it. 

* The number of comets that have occasionally appeared within the limits of 
the solar system, is variously stated from 350 to 500. The paths or orbits of about 
98 of these have been calculated from observation of the times at which they 
most nearly approached the sun ; their distance from it and from the earth at 
those times ; the direction of their movements, whether from east to west, or from 
west to east ; and the places in the starry sphere at which their orbits crossed 
that of the earth, and their inclination to it. The result is that of these 98, 24 
passed between the Sun and Mercury, 33 passed between Mercury and Venus, 21 
between Venus and the Earth, 16 between the Earth and Mars, 3 between Mars 
and Ceres, and 1 between Ceres and Jupiter; that 50 of these comets moved from 
east to west ; that their orbit* were inclined at every possible angle to that of the 
earth ; the greater part of them ascended above the orbit of the earth, when very 
near the sun ; and some were observed to dash down from the upper regions of 
space, and after turning round the sun to mount again. 

To what class of bodies is this name given ? Of what do these bodies appear 
to consist ? What is the number of comets that have occasionally appeared ? 
r What discoveries have been made concerning 98 of them? What is the result ? 
What is the form of the orbits of comets? What is said of the motion of comets 
when in perihelion? What did Newton calculate the velocity of the comet io 
1680, to be in an hour ? For what was this comet remarkable? What is said of the 
luminous stream of a comet as it approaches and recedes from the sun ? What 
did Newton, and some other astronomers, consider the tails of comets to be ? 



ASTRONOMY. 211 

The tails of comets differ very greatly in length, and some are 
attended apparently by only a small cloudy light, while the length 
of the tail of others has been estimated, at from 50, to 80 millions of 
miles.* 

477. The stars are classed into six magnitudes ; the 
largest are of the first magnitude, and the smallest that 
can be seen by the naked eye, are of the sixth. Those 
stars which can be seen only by means of telescopes, 
are called telescopic stars. 

The distance of the fixed stars cannot be determined, because 
we have no means of ascertaining the distance of any body, which 
exceeds 200 thousand times that of the earth. As none of the stars 
come within that limit, we cannot determine their real distance. 
It is generally supposed, that a part, if not all of the difference in 
their apparent magnitudes, is owing to the difference in their dis- 
tance, the smallest being farthest off. 

Although the stars generally appear fixed, they all have motion ; 
but their distance being so immensely great, a rapid motion would 
not perceptibly change their relative situation in two or three thousand 
years. Some have been noticed alternately to appear and disap- 
pear; several that were mentioned by ancient astronomers, are not 
now to be seen ; and some are now observed, which were unknown 
to the ancients. 

*It has been argued that comets consist of very little solid substance, because, 
although they sometimes approach very near to the other heavenly bodies, they 
appear to exert no sensible attractive force upon those bodies. It is said, that in 
1451 the moon was eclipsed by a comet. The comet, must, therefore, have been 
very near the earth ; (less than 240 thousand miles,) yet it produced no sensible ef- 
fect on the earth, or the moon, for it did not cause them to make any perceptible 
deviation from their accustomed paths round the sun. It has been ascertained 
that comets are .iisturbed by the gravitating power of the planets, but it does 
not appear that the planets are in like manner affected by comers. 

Many comets escape observation, because they traverse that part of the heavens 
only which is above the horizon in the day lime. They are", therefore, lost in the 
brilliance of the sun, and can be seen only when a total eclipse of the sun takes 
place. Seneca, 60 years before the Christian era, states that a large comet wa3 
actually observed very near the sun, during au eclipse. 

Dr. Halley and Professor Encke and Biela are the first, astronomers that ever 
successfully predicted the return of a comet. The periodical time of Halley's 
comet is about 76 years. It appeared last in the fall of 1835, — that, of Encke is 
about 1200 days— that of Biela about 6 3-4 years. This last comet appeared in 
1832, its next appearance will be in 1838. 

The comet of 1753, the return of which was predicted by Dr. Halley, was look- 
ed upon with great interest by astronomers, because its return was ■predicted. But 
four revolutions before, in 1456, it was looked upon with the utmost horror. Its 
long tail spread consternation over all Europe, already terrified by the rapid suc- 
cess of the Turkish arms. Pope Callixtus, on this occasion, ordered a prayer, in 
whicn both the comet and the Turks were included in one anathema. 

What is said in the note with rpgard to comets .' Who were the first astrono- 
mers that successfully predicted the return of a comet? What is the periodical 
time of Halley's comet ; OfEncke's? Of Biela's ? 477. Into how many mag- 
nitudes are the stars classed > Of what magnitude are the largest ? Of what are 
the smallest! What are telescopic stars? Why cannot the distance of the fixed 
stars be determined .' To what is the difference in their apparent magnitudes sup- 
posed to be owiug ? Have the stars any motion > 



212 NATURAL PHILOSOPHY. 

Many stars which appear single to the naked eye, when viewed 
through powerful telescopes appear double, treble, and even quad- 
ruple. Some are subject to variation in their apparent magnitude ; 
at one time being of the second, or third, and, at another, of the 
fifth or sixth magnitude. 

478. The Galaxy, or milky way, is the name given 
to a remarkably light broad zone, visible in the heav- 
ens, passing from north-east to southwest. It is sup- 
posed to consist of an immense number of stars, which, 
from their apparent nearness, cannot be distinguished 
from each other. 

Dr. Herschel saw, in the course of a quarter of an hour, the as- 
tonishing number of 116,000 stars pass through the field of his tel- 
escope, while it was directed to the milky way. 

479. The ancients, in reducing astronomy to a sci- 
ence, formed the stars into clusters, or constellations,* to 
which they gave particular names. 

The number of constellations among the ancients was about fifty. 
The moderns have added about fifty more.t 

On a celestial globe, the largest star in each constellation is usu- 
ally designated by the first letter of the Greek alphabet ; and the 
next largest by the second, &c. When the Greek alphabet is ex- 
hausted, the English alphabet, and then numbers are used. 

480. The stars, and other heavenly bodies are never 
seen in their true situation, because the motion of light 
is progressive ; and, during the time that light is coming 
to the earth, the earth is constantly in motion. In or- 
der, therefore, to see a star, the telescope must be turn- 
ed somewhat before the star, in the direction in which 
the earth moves. [See Resultant 3Iotion, page 43.] 

* The names of the signs of the zodiac have already been given. (See page 193.) 
It remains to be observed that each constellation is about 30 degrees, or a sign, 
eastward of the sign of the same name. For example, the constellation Aries is 
30° e:is'ward of the sign Aries, and the constellation Taurus, 30° eastward of the 
sign Taurus, and so on. Thus the sign Aries lies in the constellation Pisces; the 
sign Taurus in the constellation Aries ; the sign Gemini in the constellation Tau- 
rus, and so on. Hence the importance of distinguishing between the signs of the 
zodiac and the constellations of the zodiac. The cause of the difference is the 
precession of the equinoxes. [See note on page 193 and page 220.] 

| Our observations of the stars and nebula are confined principally to those of 
the northern hemisphere. Of the constellations near the south pole, we know but 
little. 

478. What is the galaxy ? Of what is it supposed to consist? 479. How did 
the ancients divide the stars? What was the number of constellations among the 
aDcients ? How many have been added by the ancients ? How are the stars des- 
ignated on the celestial globe? What is the situation of each constellation now? 
Illustrate this. What is the cause of this difference ? 480. Why do we not see 
the stars, and other heavenly bodies, in their true situation > How ean a star be 
seen in its true situation ? 



ASTRONOMY. 



213 



Hence, a ray of light passing through the centre of the telescope, 
to the observer's eye, does not coincide with a direct line from his 
eye to the star, bat makes an angle with it ; and this is termed the 
aberration of light? 

481. On account of the daily rotation of the earth 
on its axis, the whole sphere of the fixed stars, &c. ap- 
pears to move round the earth every twenty-four hours 
from east to west. To the inhabitants of the northern 
hemisphere, the immovable point, on which the whole 
seems to turn, is the Pole Star. To the inhabitants of 
the southern hemisphere, there is another, and a cor- 
responding point in the heavens. 

Certam of the stars surrounding the south pole, never set to us, 
These are included in a circle parallel with the equator, and in 
ever}- part equally distant from the north pole star. This circle is 
called the circle of perpetual apparition. Others never rise to us; 
these are included in a circle equally distant from the south pole ; 
and this is called the circle of perpetual occultation. 

Some of the constellations of the southern hemisphere, are rep- 
resented as inimitably beautiful, particularly the cross. 

482. The parallax of a heavenly body is the differ- 
ence between the true and the apparent situation of the 
body. 

Illustration. In fig. 140, A G B represent the earth, and C 
the moon. To a spectator at A, the moon would appear at F ; 
while to another at B, the the moon would appear at D ; but to a 
Fig. 140. D 




third'spectator at G, the centre of the earth, the moon would ap- 
pear^at E. which is the true situation. The ' dista o from F to E 
is the parallax of the moon when viewed from A, .ad the distance 
from^E to D is the parallax when viewed from B. 

* In determining the true place of any of the celestial bodies, the refractive 
power of the atmosphere must always be taken into consideration. This proper- 
ty of the atmosphere adds to the length of the days, by causing the sun to appear 
before it has actually risen, and by detaining its appearance after it has actually set. 

What is meant by the aberration of light ? What is necessary to be taken into 
consideration, ii. determining the true place of the celestial bodies? What effect 
has this property of the atmosphere on the length of the days; 482. What is th« 
parallax of a heavenly body ? Explain fig. 140. What appears from this >. 

19* 



214 NATURAL PHILOSOPHY. 

From this it appears, that the situation of the heavenly bodies 
must always be calculated from the centre of the earth; and the 
observer must always know the distance between the place of his 
observation, and the centre of the earth, in order to make the 
necessary calculations, to determine the true situation of the body- 
Allowance also must be made for refraction. [See note to No. 354.] 

483. The moon is a secondary planet, revolving 
about the earth, in about 29£ days. Its distance from 
the earth is about 240,000 miles. It turns on its axis 
in precisely the same time that it performs its revolu- 
tion about the earth. Consequently it always presents 
the same side to the earth. 

The most obvious fact in relation to the moon, is that its disk is 
constantly changing its appearance, sometimes only a semicircular 
edge being illuminated, while the rest is dark ; at another time, the 
whole surface appearing resplendent. This is caused by the relative 
position of the moon with regard to 'the sun and the earth. The moon 
is an opaque body, and shines only by the light of the sun. When, 
therefore, the moon is between the earth and the sun, it presents its 
dark side to the earth; while the side presented to the sun, and on 
which the sun shines, is invisible to the earth. But when the earth 
is between the sun and the moon, the illuminated side of the moon 
is visible at the earth. 

Illustration. In Fig. 141, let S be the sun, E the earth, and 
A B C D the moon in different parts of her orbit. When the 
Fig. 141. 




moon is at A, its dark side will be towards the earth, its illumi- 
nated part being always towards the sim. Hence the moon will 
appear to us as represented at a. But when it has advanced in 

What allowance must, also be made ? 4S3. Is the moon a primary or secondary 
planet? How long is it in performing its revolution about the earth I What is its 
distance from the earth ? What is the most obvious fact in relation to the moon? 
How is 1 his caused I What kind of a body is the moon ? By what light does it shine ? 



ASTRONOMY. 215 

its orbit, and come to B, a small part of its illuminated side 
comes in sight it appears as represented at b, and is said to be 
horned. When it has come to C, one half its illuminated side is 
visible, and it appears as at c. At C, and in the opposite point of 
its orbit, the moon is said to be in quadrature. At D its appear- 
ance is as represented at d, and it is said to be gibbous. At E all 
its illuminated side is toward us, and we have a full moon. 
Daring the other half of its revolution, less and less of its illumi- 
nated side is seen, till it again becomes invisible at A.* 

The mean difference in the rising of the moon, caused by its daily 
motion, is a little less than an hour. But on account of the differ- 
ent angles formed with the horizon by different parts of the ecliptic, 
it happens that for six or eight nights near the fall moons of Sep- 
tember and October, the moon rises nearly as soon as the sun is 
set. As this is a great convenience to the husbandman and the 
hunter, inasmuch as it affords them light to continue their occupa- 
tion, and, as it were, lengthens out their day, the first is called the 
harvest moon, the second the hunter's moon. These moons are al- 
ways most beneficial when the moon's ascending node is in or near 
Aries A 

484. An eclipse is a total or partial obscuration of 
one heavenly body by the intervention of another. J 

* The following signs are used in our common almanacs to denote the different 
positions and phases of the moon. ~) or 5 denotes the moon in the first quadra- 
ture; that is, the quadrature between change and full ;C or fj denotes the moon in 
the last quadrature; that is, the quadrature between full and change ; O denotes 
new moon ; © denotes full moon. 

When viewed through a telescope, the surface of the moon appears wonderfully 
diversified. Large dark spots, supposed to be excavations or valleys, are visible to 
the eye; some parts also, appear more lucid than the general surface. These 
are ascertained to be mountains, by the shadows which they cast. Maps of the 
moon's surface have been drawn ; on which most of these valleys and mountains 
are delineated, and names are given to them. Some of these excavations are 
thought to be 4 miles deep and 40 wide. A high ridge generally surrounds them, 
and often a mountain rises in the centre. These immense depressions probably 
very much resemble what would be the appearance of the earth at the moon, were 
all the seas and lakes dried up. Some of the mountains are supposed to be volcanic. 

f The Teader who wishes a simple and clear illustration of the causes which 
produce the harvest moon is referred to Wilkius' Astronomy, page 69. 

J The situation of the earth with regard to the moon, or rather of the moon 
with regard to the earth, occasions eclipses both of the sun and moon. Those of 
the sun take place when the moon, passing between the sun and earth, intercepts 
his rays. Those of the moon take place when the earth, coming between the sun 
and moon, deprive the moon of his light. Hence, an eclipse of the sun can take 
place only when the moon changes, and an eclipse of the moon only when the 
moon fulls; for at the time of an eclipse, either of the sun or moon, the sun, earth, 
and moon must he in the same straight line. 

If the moon went around the earth in the same plane in which the earth goes 



How does the moon appear when viewed through a telescope ? What causes 
the difference in the rising of the moon ? What is the mean difference in the ris- 
ing of the moon ? What is the harvest moon ? What is the hunter's moon ? When 
are the moons always the most beneficial >. 484. What is an eclipse ? When does 
an eclipse of the sun take place ? When does an eclipse of the moon take place I 
What is necessary at the time of an eclipse I 



216 



NATURAL PHILOSOPHY. 



485. The tides are the regular rising; and falling: of 
the water of the ocean twice in about 25 hours. They 
are occasioned by the attraction of the moon; but are 
affected by that of the sun also. 

Fig. 142. 





Let M, in the above figure, be the moon revolving in its orbit ; 
E, the earth covered with water. The moon, attracting the earth, 
affects the solid parts of it, as if its whole weight were in a point at 

around the sun, that is, in the ecliptic, it is plain that the sun would be eclipsed at 
every new moon ; and the moon would be eclipsed at every full. For at each of 
these times, these three bo;lies would he in the same straight line. But the moon's 
orbit does not coincide with the ecliptic, but is inclined to it at an angle of about 
5° 20'. Hence, since the apparent diameter of the sun is but about 1-2 a degree, 
and that of the moon about the same, no eclipse will take place at new or full 
moon, unless the moon be wilhin ] 2 a degree of the ecliptic, that is, in or near 
one of its nodes. It is found that if the moon be wilhin 16 1-2 Q of a node at time 
of change, it will be so near the ecliptic, that the sun will be more or less eclipsed ; 
if within 12° at time of full, the moon will be more or less eclipsed. 

It is obvious that the moon will he oftener within 16 1 -2 a of a node at the time 
of change, than within 12° at the time of full ; consequently there will be more 
eclipses of the sun than of the moon in a course of years. As the nodes common- 
ly come between the sun and earth but twice in a year, and the moon's orbit con- 
tains 3C0°, of which 161-2°, the limit of solar eclipses, and 21°, the limit of lunar 
eclipses, are but small portions, it is plain there must be many new and full moons 
without any eclipses. 

Although there are more eclipses of the sun than of the moon, yet more eclipses 
of the moon will be visible at a particular place, as Boston, in a course of years, 
than of the sun. Since the sun is very much larger than either the earth or moon, 
the shadow of these bodies must always terminate in a point; that is, it must al- 
ways be a cone. In Fig. 143, let S be the sun, m the moon, and E the earth. The 
Fig. 143. 




485. What are tides ? By what are they occasioned ? Explain fig. 143. How 
often would there be an eclipse, if the moon went round the earth in the same 
plane in which the earth goes round the sun ? Why ? What is the inclination of 
the moon's orbit to the ecliptic i What is the apparent diameter of the sun and 
moon ? What follows from this » When is the sun eclipsed ? When the moon ' 



Does an eclipse happen every time there 
shadows of these bodies always be ? Why 



full or new ruoon ? What must, the 
Explain fig. 143. 



ASTRONOMY. 



217 



or near the centre E. Bnt the waters at A, being nearer the moon 
than the point E, are more strongly attracted than the earth, 
at E, and are consequently drawn away from the earth, and raised 
up under the moon at A. The waters, on the opposite side at B, 
being farther from the moon than the earth at E, are consequently 
less powerfully attracted than the earth, which is drawn from them, 
and they are raised at B. When the waters are raised at A and 
B, it is plain they must recede from the intermediate points C and D. 
Thus any particular place as A, while passing from under the 
moon, till it comes under the moon again, has two tides. But the 
moon' is constantly advancing in its orbit, so that the earth must 
a little more than complete its rotation, before the place A comes 
under the moon. This causes high water at any place about 50 
minutes later each successive day. 

sun constantly illuminates half the earth's surface, that is, a hemisphere ; and 
consequently it is visible to all in this hemisphere. But the moon's shadow falls 
upon a part only of this hemisphere ; and hence the sun appears eclipsed to a part 
only of those to whom it is visible. Sometimes, when the moon is at its greatest 
distance, its shadow, O m, terminates before it reaches the earth. In eclipses of 
this kind, to an inhabitant directly under the point O, the outermost edge of the 
sun's disk is seen, forming a bright ring round the moon ; from which circum- 
stance these eclipses are called annular, from annulus, a Latin word for ring. 

Besides the dark shadow of the moon, m O, in which all the light of the sun is 
intercepted, (in which case the eclipse is called total,) there is another shadow, r 
CDS, distinct from the former, which is called the penumbra. Within this, only 
a part of the sun's rays are intercepted, and the eclipse is called partial. If a per- 
son could pass, during an eclipse of the sun from O to D, immediately on immerg- 
ing from the dark shadow, O m, he would see a small pait of the sun ; and would 
continually see more and more till he arrived at D, where all shadow would cease, 
and the whole sun's disk be visible. Appearances would be similar if he went 
from O to C. Hence the penumbra is less and less dark, (because a loss portion of 
the sun is eclipsed.) in proportion as the spectator is more lemole fiom O, and 
nearer C or D. Though the penumbra is cominually increasing in diameter ac- 
cording to its length, or the distance of the moon from the eanh, still, under the 
most favorable circumstances, it, falls on but about half of the illuminated hemis- 
phere of the earth Hence, by half the inhabitants on this hemisphere, no eclipse 
will be seen. 

Fig. 144 represents an eclipse of the moon. The instant the moon enters the 
earth's shadow at z, it is deprived of the sun's light, and is eclipsed to all in the 
unilluminated hemisphere of the earth. Hence, eclipses of the moon ate visible to 
at least twice as many inhabitants as those of the sun can be ; generally the pro- 



Fig. 144. 



R 




When is an eclipse called annular ? Explain by fig. 144 What is a penumbra i 
Why are eclipses of the moon visible to more inhabitants than those of the sun i 



218 NATURAL PHILOSOPHY. 

As the moon's orbit varies but little from the ecliptic, the moon is 
never more than 29 G from the equator, and is generally much less. 
Hence the waters about the equator being nearer the moon, are 
more strongly attracted, and the tides are higher than towards the 
poles. 

The sun attracts the waters as well as the moon. "When the 
moon is at full or change, being in the same line of direction, it acts 
with the sun ; that is, the sun and moon tend to raise the tides at 
the same place, as seen in the figure. The tides are then vetyhigh, 
and are called spring tides. 

portion is much greator. Thus, the inhabitants at a particular place, as Eoston, 
see more eclipses of the moon than of the sun. 

The reason why a lunar eclipse is visible to all to whom the moon at the time 
is visible, and a solar one is not to all to whom the sun at the time is visible, may 
be seen from the nature of these eclipses. We speai; of the sun's being eclipsed ; 
but properly i! is the earth which is eclipsed. No change takes place in the sun ; 
if there were, it would be seen by all to whom the sun is visihle. The sun contin- 
ues to diffuse its beams as freely and uniformly at such times as at others. But 
these beams are intercepted, and the earth is eclipsed only where the moon's 
shadow falls, that is, on only a part of a hemisphere. In eclipses of the moon, 
that body ceases to receive light from the sun, and, consequently, ceases to reflect 
it to the earth. The moon undergoes a change in its appearance ; and, conse- 
quently this change is visible at the same time to all to whom the moon is visible ; 
that is, to a whole hemisphere of the earth. 

The earth's shadow (like that of the moon) is encompassed by a penumbra, C R 
S D, which is faint at the edges towards R and S, but becomes darker towards F 
and G. The shadow of the earth is but little darker than the region of the pe- 
numbra next to it. Hence it is very difficult to determine the exact time when the 
moon passes from the penumbra into the shadow, and from the shadow into the 
penumbra; that is, when the eclipse begins and ends. But the beginning and 
ending of a solar eclipse may be determined instantaneously. 

The diameters of the sun and moon are supposed to be divided into 12 equal 
parts, called dibits. These bodies are said to have as many digits eclipsed as there 
are of those parts involved in darkness. 

There must be an eclipse of the sun as often at least as one of the moon's nodes 
conies between the sun and the earth. 

The greatest number of both solar and lunar eclipses that can take place during 
a year is seven. The usuil number is four ; two solar and two lunar. 

A total eclipse of the sun is a very remarkable phenomenon. 

June 16", 1806, a very remarkable total eclipse took place at Boston. The day 
was clear, and nothing occurred to prevent accurate observation of this interest- 
ing phenomenon. Several stars were visible ; the birds were greatly agitated; a 
gloom spread over the landscape, and an indescribable sensation of fear or dread 
pervaded the breasts of those, who gave themselves up to thesimple effects of the 
phenomenon, without having their attention diverted by efforts of observation. 
The first gleam of light, contrasted with the previous darkness, seemed like the 
usual meridian day, and gave indescribable life and joy to the whole creation. A 
total eclipse of th^» sun can last but little more than "three minutes. An annular 
eclipse of the sun is still more rare than a total one. 



What, is the distance of the moor, from the equator? Where are the tides 
the highest ? Why? How are spring tides caused >. Why is a lunar eclipse vis- 
ible to all to whom the moon is visible at the time >. What is said of the earth's 
shadow? Explain by the figure i Into what are the diameters of the sun and 
moon supposed to be divided ? How many digits are these bodies said to have 
eclipsed; How often must there be an eclipse of the sun » What is i he greatest 
number, of both lunar and solar eclipses, that can take place during a year? 
What is the usual number ? What is said of the eclipse of the sun in 1806 ? 



ASTRONOMY. 219 

But when the moon is in its quarters, as in figure 145, the sun 
and moon being in opposite directions, tend to raise tides at different 
Fig. 145. 





places ; namely, the moon at C and D, and the sun at A and B. 
Tides, that are produced when the moon is in its quarters, are low, 
and are called neap tides-* 

486. When time is calculated by the sun, it is called 
solar time, and the year a solar year; but when it is 
calculated by the stars,f it is called sidereal time, and 
the year a sidereal year. The sidereal year is 20 min- 
utes and 24 seconds longer than the solar year. 

A solar year* is measured from the time the earth sets out from a 
particular point in the ecliptic, as an equinox, or solstice, until it re- 
turns to the same point again. A sidereal year is measured by the 
time that the earth takes in making an entire revolution in its orbit ; 

*There are so many natural difficulties to the free progress of the tides, that 
the theory by winch they are accounted for, is, in fact, and necessarily, the most 
imperfect of all 'he theories connected with astronomy. It is, however, indisput- 
able that the moon has an effect upon the tides, although it is not equally felt in 
all places, owing to the indentations of the coast — the obstructions of islands, 
continents, &:c, which prevent the free motion of the waters. In narrow rivers, 
the tides are frequently very high and sudden, from the resistance afforded by their 
banks to the free ingress of the water, whence what would otherwise be a tide 
becomes an accumulation. It has been constantly observed that the spring tides 
happen at the new and full moon, and the neap tides at the quarters. This cir- 
cumstance is sufficient in itself to prove the connexion between the influence of 
the moon and the tides. 

t The solar year consists of 365 days, 5 hours, 4S minutes, and 48 seconds, but 
our common reckoning gives 365 days only to the year. As the difference amounts 
to nearly a quarter of a day, every year, it is usual every fourth year to add a day. 
Every fourth year, the Romans reckoned the 6th of the calends of March, and the 
following day as one day ; which on that account they called bissextile, or twice 
the 6th day ; whence we derive the name of bissextile, for the 1 sap year, in which 
we give to February, for the same reason, 29 days every fourth year. 

X As it may be interesting, to those who have "access to a celestial globe, to know 
how to find any particular star or constellation, the following directions are sub- 
joined : — 

There is always to be seen, on a clear night, a beautiful cluster of se% r en bril- 
liant stars, which belong to the constellation " Una. Major," or the Great Bear. 
Some have supposed that they will aptly represent a plough — others say that they 

How are neap tides caused ? Explain fig. 145. When do spring tides happen ? 
When, neap tides ? 486. What is time called when calculated by the sun ? What 
js sidereal time? How much longer is the sidereal year than the solar. How is a 
solar year measured. What, is the length of a solar year ? Why is a day added 
every fourth year, to the year? How is a sidereal year measured? 



220 



NATURAL PHILOSOPHY. 



or, in other words, from the time that the sun takes to return in 
conjunction with any fixed star. 

Every equinox, happens 50 seconds of a degree of the great cir- 
cle, preceding the place of the equinox, 12 months before ; and this 
is called the precession of the equinoxes. It is this circumstance 
which has caused the change in the situation of the constellations 
mentioned in pages 193 and 212. 

The earth's diurnal motion on an inclined axis, together with its 
annual revolution in an elliptic orbit, occasions so much complica- 
tion in its motion, as to produce many irregularities ; therefore true 
equal time cannot be measured by the sun. A clock, which is 
always perfectly correct, will in some parts of the year be be- 
fore the sun, and in other parts after it. There are but four periods 
in which the sun and a perfect clock will agree ; these are the 
15th of April, the 16th of June, the 23d of August, and the 24th 
of December. 

The greatest difference between true and apparent time, amounts 
to between fifteen and sixteen minutes. Tables of equation are 
constructed for the purpose of pointing out and correcting these 
differences between solar time and equal or mean time, the de- 
nomination given by astronomers to true time. 

are more like a waggon and horses— the four stars representing the hody of the 
waggon, and the other three the horses. Hence, they are called by some the plough, 
and by others they are called Charles' 1 wain, or waggon. 



Fi-. 



116. 



^€b 



B/ 



7z p. i v* 



Fig. 146 represents these seven stars ; a b gd 
represent the four, and e z B the other threo 
stars. Perhaps they may more properly be call- 
ed a large dipper, of which e z B represent the 
handle. If a line be drawn through the stars b 
and a and carried upwards, it will pass a little 
to the left and nearly touch a star represented 
in the figure by P. This is the polar star, or 
the North pole star, and the stars band a, which 
appear to point to it, are called the pointers, 
because they appear to point to the polar star. 

The polar star shines with a steady and rath- 
er dead kind of light. It always appears in the 
same position ; and the north pole of the earth 
> always points to it at all seasons of the year. 
The other stars seem to move round it as a cen - 
tre. As this star is always in the north, the 
cardinal points may at any time be found by 
starlight. 

By those stars we can also find any other 
star or constellation. 

A. Thus, if we conceive a line drawn from the 

star z, leaving B a little to the left, it will pass 
through the very brilliant star A. By looking 
on a celestial globe for the star z, and supposing the line drawn on the globe, as we 
conceive it done on the heavens, we shall find the star and its name, which is 
Arcturus. 

Conceiving another line, drawn through g and b, and extended some distance to 
to the right, it will pass just above another very briiiiant star. On referring to 
the globe we find it to be Capella, or the goat. 

In this manner the student may become acquainted with the appearance of the 
whole heavens. 

What is the precession of the equinoxes ? What change has this circumstance 
caused with regard to the situation of the constellations i Can true, equal time 
be measured by the sun? Why? At what periods of the year do the sun and a 
perfect clock agree .' What is the greatest difference between true and apparent 
time. 



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INDEX. 



[The figures refer to the page.] . 

Attraction, 13 

Attraction of Cohesion, 14. 

Action, 30. 

Archimedes' discovery of specific gravity, 71 ; screw of, 74. 

Air, how high it extends, &c, 79 ; elasticity of, 79 ; pressure of, 89, 
how it becomes a mechanical agent, 89; made solid, 107; [note.] 

Air Pump, 80, experiments with, 82, &c ; and instruments connect- 
ed with, 82, &c. 

Acoustics, 97. 

Angle, right obtuse and acute, 32 ; of vision, 125 ; of incidence and 
reflection, 33, and 127. 

Aqueous humor, 140. 

Amalgam, 157. 

Aurora Borealis, 165. 

Armature of a magnet, 180. 

Ampere, his apparatus for illustrating the electro-magnetic rota- 
tion, 186. 

Astronomy, 188. 

Aphelion, 195 ; Aphelia of the planets, in what sign, 195. 

Apogee, 195. 

Axis of the planets, their inclination, &c, 196. 

Aberration of light, 213. 

Asteroids, 206 ; supposed to be fragments of a large planet burst 
asunder, 207. 

Brittleness, 19. 

Barometer, 92 and 93. 

Battery, electrical, 156. 

Biela's comet, 211. 

Bissextile, or leap year, 219. 

Compressibility, 17. 

Compound motion, 34. 

Clock, how regulated, 45. 

Cylinder, 51. 

Complex wheel work, 53. 

Capstan, 55. 



224 INDEX. 

Crank, 55, 

Caloric, 104. 

Catoptrics, 127 ; fundamental law of, 130. 

Cornea, 139. 

Crystalline lens, 140. 

Choroid, 141. 

Chromatics, 148. 

Color, cause of, 148 and 150. 

Compound galvanic battery, 172. 

Couronne des tasses, 172. 

Calorimotor, 171. 

Constellations of the Zodiac, 193 ; ancient, 212. 

Conjunction, inferior and superior, 195. 

Characters used in Astronomical works, 196. [Note.] 

Circles on the earth, 201 ; of perpetual apparition and occupation, 213. 

Ceres, 206. 

Comets, 209. 

Clusters of stars, 212. 

Celestial globe, how used, 219. 

Divisibility, 9. 

Density, 15. 

Ductilitv, 19. 

Diving Bell, 91. 

Dioptrics, laws of, 134. 

Dipping of magnetic needle, 178. 

Distance of the planets from the sun, 191. 

Diameter, 32. 

Diagonal, 33. 

Days and nights, cause of their different lengths, 203. 

Digits, 218. 

Extension, '9. 

Expansibility, 18. 

Elasticity, 18. 

Equilibrimn of fluids, 62. 

Echo, how produced, 100. 

Eye, its parts and description of, 139 and 141. 

Electricity, 152; vitreous and resinous or positive and negative, 154; 
conductors of, 153 ; by induction and transfer, 157 and 166 ; elicit- 
ed from a magnet, 187. 

Electrics, 153. 

Electrical machine, 157; experiments with, 159; electrical bells, 

161 ; electrical sportsman, 163; electrical saw-mill, l65[note;] elec- 
trical animals, 167. 

Electrometer, 159. 

Electro-Magnetism, 180 ; principal facts relating to the science, 182; 
remarks on the science, 188. 

Electric sparks taken from a magnet, 181. 

Electro-Magnetic multiplier, 184 ; rotation, 184. 

Earth, its diameter, &c. 188. 

Ecliptic, 193. 

Earth, not a perfect sphere ; appears as a moon to the inhabitants of 
the moon, &c. 206. 

Encke's comet, 211. 



INDEX. 225 

Eclipses, 215; total eclipse of the sun in 1806, 218. 

Equinoxes, procession of, 220. 

Errata, 7*. 

Figure, 9. 

Force, central, 35 ; centripetal and centrifugal, 35. 

Fulcrum, 46. 

Fly-wheel, 55. 

Friction, 58. 

Fluids, 61 ; pressure of, 65. 

Fountain, how formed, 76. 

Farraday's discoveries, 181 and 187; his apparatus for exhibiting 
the electro-magnetic rotation, 185. 

Gravity, or Gravitation, 19 ; effect of, on fluids, 64; specific grav- 
ity, 23 ; centre of, 36. 

Governor, 60. 

Glass chimneys, how protected from fracture, 109. [Note.] 

Gasometer, or gas generator, 162. 

Gymnotus Electricus, 167. 

Galvanism, 168; difference between electricity and galvanism, 168 

and 174 ; its effects 178. 
Galvanic conductors, 169. 

Galvanic circle, 169; effects of, how increased, 171. 

Galvanometer, 184. 

Georgium Sidus, 209. 

Galaxy, 212. 

Gibbous,when the moon appears, 215. 

Great Bear, 219. 

Heat, its effects, 17; laws of, 104; sources and effects of, 105; when 

greatest on the earth,, 198; how propagated and reflected, il8. 
Hydrostatics, 61. 
Hydrostatic Bellows, 67. 
Hydrostatic Press, (Bramah's,) 68. 
Hydrometer, 71. 
Hydraulics, 72. 
Hygrometer, 92. 
Harmony, science of, 98. 

Heavenly bodies, why not seen in their real place, 135, their situa- 
tion must be calculated from the centre of the earth, 214; cause of 
their motion, 198. 
Hydrogen pistol, 162. 
Hydro-electric current, 181. 
Helix, 184. {Note.] 
Hesperus, 205. [Note.] 
Herschel, 209. 
Halley's comet, 211. 
Harvest moon, 215. 
Impenetrability, 10. 
Indestructibility, 11. 
Inertia, 12. 

Incident motion, 32 ; incident ray, 127. 
Incidence, angle of, 32, 
Inclined plane, 56. 
Iris, 140. 
Insulated, 153. 

20* 



226 INDEX. 

Induction, electricity by, 157. 

Juno, 207. 

Jupiter, 207; his satellites, &c, 207. 

Kaleidescope, 151. 

Kepler's Law, 196 ; illustration of, 197. 

Lever, 46. 

Liquids, 61. 

Locomotive steam engine, 116. 

Light, laws of, 121; composed of different colors, 148; its velocity, 
how ascertained, 208 ; reflected light, laws of, 130. 

Luminous bodies, 119. 

Lens, various kinds of, 136 ; focal distance of, 137 ; effects of, 137 ; 
(note,) why used in spectacles, 138. 

Leyden jar, 155; how, silently discharged, 157. 

Lightning, 165. 

Lightning rods, 157; square, better than round ones, 166; (note,) 
must not be painted, 166 ; (note.) Dr. King's and Mr. duimby's, 
166, (note,) ; first proposed by Franklin, 167. 

Loadstone, 175. 

Lucifer, 205. 

Longitude ascertained by eclipses of Jupiter's Satellites, 208. 

Matter, definition and properties of, 6. 

Mobility, 18. 

Malleability, 19. 

Mechanics, 25. 

Motion, 25 ; uniform accelerated, perpetual and retarded, 27; com- 
pound, 34; circular, centre of, axis of, 35; resultant motion, 43, 
when imperceptible, 126 ; cause of, in the heavenly bodies, 190, 
their motion not uniform, 196. 

Momentum, 29. 

Magnitude, centre of, 36. 

Mechanical powers, 45. 

Medium, 59, and 121. 

Main spring of a watch, 59. 

Magdeburgh cups, or hemispheres, 87. 

Mirrors, plain, concave and convex, 128 ; laws of reflection from, 
130; concave, why they magnify, 130; convex, why they dimin- 
ish, 130; 

Microscope, single and double, 143 ; solar 144. 

Magic lantern, 145. 

Multiplying glass, 151. 

Magnetism, 175 ; how it resembles, and differs from electricity, 177; 
communicated by electricity, 184, and 187. 

Magnet, properties of, 175; polarity of, 175 ; methods of support- 
ing, 176, its powers, how increased, 177; horse-shoe magnet, 177; 
artificial magnets, how made, 179 and 180; magnets made by 
electricity, 187. 

Mariner's compass, 179. 

Magneto-electrical machine, (Saxton's,) 187. [Note.] 

Mercury, the planet, 204, &c. 
Mars, 206. 

Meteoric stones, 207. 
Milky way, 212. 



INDEX. 22 7 

Moon, 214. 

Natural philosophy, definition of, principal branches of, 5. 
Non-electrics 153. 
Northern lights, 165. 
Oil, effects of, on waves, 74. 
Optics, 119. 
Opiic nerve, 141. 
Oersted's discoveries, 181. 
Orbits of the planets, 191. 
Opposition, 195. [Note.] 
Perpendicular, 32. 
Parallelogram, 33. 
Projectile, 38. 
Parabola, 39. 
Projectile, random of, 39. 
Pendulum, 43. 

Pulleys, 49 ; fixed and movable, 49 ; practical use of, 51. 
Pinion, 53. 
Pyronomics, 104. 
Pyrometer, 108. 
Pupil of the eye, 140. 
Prism, 148. 

Planets of the solar system, 189 ; how distinguished from stars, 189; 
interior and exterior, inferior and superior, 191 ; inhabited, 203. 
[Note.] 

Perihelion, 195. 

Perigee, 195. 

Phosphor, 205. [Note.] 

Pallas, 207. 

Pole or Polar star, 213 ; how to find, 220. 

Parallax, 213. 

Quadrature, 215. 

Rarily, 15. 

Reaction, 30. 

Reflected motion, 31. 

Radii, 32. 

Reflection, angle of, 33. 

Reflecting and refracting substances, 120. 

Reflected ray, 127. 

Refraction of light, 133 ; effects of, laws of, 138. 

Retina, 141. 

Rainbow, how produced, 150. 

Resinous electricity, 154. 

Revolution, annual, of the planets, 191 ; around their axes, 192. 

Rays, oblique and vertical, effects of, 198, and 199. 

Receiver of an air pump, 80 ; straight receiver, 163. 

Square, 33. 

Screw, 57. 

Specific gravity, 23 ; standard of, 69 ; table of specific gravities, 69, 
{note) ; how ascertained, 70 and 71 . 

Springs, how formed, 75. 
Syphon, 76. 

Sound, 97; produced by strings, 99; velocity of, 100; of the human 
voice, how produced, 102. 



228 INDEX. 

Sonorous bodies, 98. 
Steam, elastic force of, 110i 

Steam engine, 111 ; moving part of, 112; inventors and improvers 
of, 113; Watt's Steam Engine, 114; Locomotive Steam Engine,116c 
Shadows, 121, &c. 
Sclerotica, 139 and 141. 
Sky, cause of its blueness, 149. 
Spiral tube, 162. 
Straight receiver, 163. 
Silurus Electricus, 167. 
Stereo-electric current, 181. 
Saxton, J. his magneto-electrical machine, 187. 
Solar system, 189 ; tables of, 221, 222. 
Stars, 189 ; how distinguished from planets, 189 ; classed into six 

magnitudes, 211 ; never seen in their true situation, 212. 
Size, relative of heavenly bodies, 192. 
Seasons, cause of the variations of, 200 and 202. 
Sun, its size, diameter, &c. 204. 
Saturn, 208. 
Tables, 221, 222. 
Tenacity, 19. 
Tackle and fall, 51: 
Toggle joint, 61. 
Tantalus' cup, 77. 
Thermometer, 92 and 93. 
Transparent and translucent substances, 119. 
Telescopes, refracting and reflecting, 146. 
Transfer, electricity by, 157. 
Thunder-house, 164. 
Torpedo. 167. 

Thermo-electric current, 181. 
Tropic, meaning of, 201. 
Tangent, 33. 

Transit of Mercury and Venus, 205. 
Telescopic stars, 211. 
Tides, 216. 
Time, solar and sidereal, 219 ; true and apparent, difference be- 

tween, 220. 
Universal discharger, 160. 
Uranus, 209- 
Ursa Major, 219. 
Velocity, 26. 

Vibrations of a pendulum, 44. 
Velocity of a current, how ascertained, 73. 
Vacuum, 81. 
Ventriloquism, 103. 
Vision, 125. 

Vitreous humor, 140 ; vitreous'electricity, 154. 
Voltaic electricity, or galvanism, 168; difference between this and 

common electricity, 173. 
Voltaic battery, 172; effects of, &c. 173. 
Voltaic pile, 171= 
Venus, 205. 
Vesper, 205. [Nate.] 



INDEX. 229 

Vesta, 206. 

Watch, how it differs from a clock, 45. 

Wheel and axle, 51. 

Wedge, 56. 

Water, compressible, 61, {note); instruments for raising, 74 ; how it 

becomes a mechanical agent, 77. 
Water Level, 63. 
Waves, how formed, 74. 

Water wheels, overshot, undershot and breast, 77. 
Wind, 95. 

Whispering galleries, 101. 
Year, solar and sidereal, 219. 
Zodiac, 193. 
Zodiacal light, 204. 



N. B. For the convenience of recitation, the figures are all re- 
peated on separate leaves. 



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